Edges of Graph incorrectly hidden behind Image when idle

## Description of bug / unexpected behavior
First add an image to the scene. Then add a [`Graph`](https://docs.manim.community/en/stable/reference/manim.mobject.graph.Graph.html) (in the graph-theoretical sense) over it.
The graph should be in front since it was added later. However, only the vertices are in front - the edges are not.
When we animate the graph (e.g. move it around), the edges are in front as expected.
As soon as the graph is not involved in an animation, the edges disappear again.

A hacky workaround is to involve the graph in every animation, e.g. by doing `g.scale(1)`.

## Expected behavior

The graph's edges should be in front of the image at all times.

## How to reproduce the issue

Code for reproducing the problem

```py
from manim import *

class GraphImageBug(Scene):
def construct(self):
n = 256
imageArray = np.uint8([[i * 256 / n for i in range(0, n)] for _ in range(0, n)])
image = ImageMobject(imageArray).scale(2)

vertices = [1, 2, 3, 4]
edges = [(1, 2), (2, 3), (3, 4), (1, 3), (1, 4)]
g = Graph(vertices, edges)
self.add(image)
self.add(g)

self.wait()
self.play(
g[1].animate.move_to([1, 1, 0]),
g[2].animate.move_to([-1, 1, 0]),
g[3].animate.move_to([1, -1, 0]),
g[4].animate.move_to([-1, -1, 0]),
)
self.wait()

self.play(g.animate.scale(3))
self.wait()
```

## Additional media files


## Logs
Terminal output

```
[16:01:51] ~/p/rubiks-cube-video> manim -ql -p graph_image_bug.py -v DEBUG
Manim Community v0.12.0

[11/17/21 16:01:55] DEBUG Animation with empty mobject animation.py:165
DEBUG Hashing ... hashing.py:344
DEBUG Hashing done in 0.071008 s. hashing.py:356
DEBUG Hash generated : 3163782288_3493041802_919544174 hashing.py:359
DEBUG List of the first few animation hashes of the scene: ['3163782288_3493041802_919544174'] cairo_renderer.py:83
[11/17/21 16:01:56] INFO Animation 0 : Partial movie file written in '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/partial_m scene_file_writer.py:510
ovie_files/GraphImageBug/3163782288_3493041802_919544174.mp4'
DEBUG Hashing ... hashing.py:344
DEBUG Hashing done in 0.047561 s. hashing.py:356
DEBUG Hash generated : 2201830969_3362228145_1822438392 hashing.py:359
DEBUG List of the first few animation hashes of the scene: ['3163782288_3493041802_919544174', '2201830969_3362228145_1822438392'] cairo_renderer.py:83
INFO Animation 1 : Partial movie file written in '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/partial_m scene_file_writer.py:510
ovie_files/GraphImageBug/2201830969_3362228145_1822438392.mp4'
DEBUG Animation with empty mobject animation.py:165
DEBUG Hashing ... hashing.py:344
DEBUG Hashing done in 0.056231 s. hashing.py:356
DEBUG Hash generated : 2201830969_398514950_3400543663 hashing.py:359
DEBUG List of the first few animation hashes of the scene: ['3163782288_3493041802_919544174', '2201830969_3362228145_1822438392', cairo_renderer.py:83
'2201830969_398514950_3400543663']
INFO Animation 2 : Partial movie file written in '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/partial_m scene_file_writer.py:510
ovie_files/GraphImageBug/2201830969_398514950_3400543663.mp4'
DEBUG Hashing ... hashing.py:344
DEBUG Hashing done in 0.064292 s. hashing.py:356
DEBUG Hash generated : 2201830969_2942809912_3680414427 hashing.py:359
DEBUG List of the first few animation hashes of the scene: ['3163782288_3493041802_919544174', '2201830969_3362228145_1822438392', cairo_renderer.py:83
'2201830969_398514950_3400543663', '2201830969_2942809912_3680414427']
[11/17/21 16:01:57] INFO Animation 3 : Partial movie file written in '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/partial_m scene_file_writer.py:510
ovie_files/GraphImageBug/2201830969_2942809912_3680414427.mp4'
DEBUG Animation with empty mobject animation.py:165
DEBUG Hashing ... hashing.py:344
DEBUG Hashing done in 0.092485 s. hashing.py:356
DEBUG Hash generated : 2201830969_398514950_109819156 hashing.py:359
DEBUG List of the first few animation hashes of the scene: ['3163782288_3493041802_919544174', '2201830969_3362228145_1822438392', cairo_renderer.py:83
'2201830969_398514950_3400543663', '2201830969_2942809912_3680414427', '2201830969_398514950_109819156']
INFO Animation 4 : Partial movie file written in '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/partial_m scene_file_writer.py:510
ovie_files/GraphImageBug/2201830969_398514950_109819156.mp4'
INFO Combining to Movie file. scene_file_writer.py:604
DEBUG Partial movie files to combine (5 files): ['/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/partial_mo scene_file_writer.py:547
vie_files/GraphImageBug/3163782288_3493041802_919544174.mp4', '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bu
g/480p15/partial_movie_files/GraphImageBug/2201830969_3362228145_1822438392.mp4', '/Users/vaclav/prog/rubiks-cube-video/media/v
ideos/graph_image_bug/480p15/partial_movie_files/GraphImageBug/2201830969_398514950_3400543663.mp4', '/Users/vaclav/prog/rubiks
-cube-video/media/videos/graph_image_bug/480p15/partial_movie_files/GraphImageBug/2201830969_2942809912_3680414427.mp4', '/User
s/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/partial_movie_files/GraphImageBug/2201830969_398514950_1098
19156.mp4']
INFO scene_file_writer.py:718
File ready at '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/GraphImageBug.mp4'

INFO Rendered GraphImageBug scene.py:234
Played 5 animations
INFO Previewed File at: '/Users/vaclav/prog/rubiks-cube-video/media/videos/graph_image_bug/480p15/GraphImageBug.mp4' file_ops.py:202
```

## System specifications

System Details

- macOS 10.15.7 (Catalina)
- RAM: 8GB
- Python version: 3.9.7
- Installed modules (provide output from `pip list`):
```
Package Version Location
------------------------- --------- ------------------------
absl-py 0.13.0
appdirs 1.4.4
appnope 0.1.2
backcall 0.2.0
black 20.8b1
cachetools 4.2.2
certifi 2021.5.30
chardet 4.0.0
click 7.1.2
click-default-group 1.2.2
cloup 0.7.1
colorama 0.4.4
colour 0.1.5
commonmark 0.9.1
Cython 0.29.24
decorator 5.1.0
distlib 0.3.2
filelock 3.0.12
glcontext 2.3.4
google-auth 1.31.0
google-auth-oauthlib 0.4.4
grpcio 1.38.0
idna 2.10
ipython 7.22.0
ipython-genutils 0.2.0
isosurfaces 0.1.0
jedi 0.18.0
joblib 1.1.0
kociemba-manim-rubikscube 0.0.1
manim 0.12.0
manim-rubikscube 0.1.0
ManimPango 0.3.0
mapbox-earcut 0.12.10
Markdown 3.3.4
moderngl 5.6.4
moderngl-window 2.4.0
multipledispatch 0.6.0
mypy 0.812
mypy-extensions 0.4.3
networkx 2.6.3
numpy 1.20.2
oauthlib 3.1.1
packaging 20.9
pandocfilters 1.5.0
parso 0.8.2
pathspec 0.8.1
pexpect 4.8.0
pickleshare 0.7.5
Pillow 8.3.2
pip 21.2.4
prompt-toolkit 3.0.18
protobuf 3.17.3
ptyprocess 0.7.0
pyasn1 0.4.8
pyasn1-modules 0.2.8
pycairo 1.20.1
pydub 0.25.1
pygame 2.0.1
pyglet 1.5.21
Pygments 2.8.1
pyobjc-core 7.3
pyobjc-framework-Cocoa 7.3
pyparsing 2.4.7
PyQt6 6.1.0
PyQt6-3D 6.1.0
PyQt6-Charts 6.1.0
PyQt6-DataVisualization 6.1.0
PyQt6-NetworkAuth 6.1.0
PyQt6-sip 13.1.0
pyrr 0.10.3
regex 2021.3.17
requests 2.25.1
requests-oauthlib 1.3.0
rich 10.11.0
rsa 4.7.2
scikit-learn 1.0
scipy 1.7.1
screeninfo 0.6.7
setuptools 57.4.0
sip 6.1.1
six 1.16.0
skia-pathops 0.7.1
TBB 0.2
tensorboard 2.5.0
tensorboard-data-server 0.6.1
tensorboard-plugin-wit 1.8.0
termcolor 1.1.0
threadpoolctl 3.0.0
toml 0.10.2
tqdm 4.59.0
traitlets 5.0.5
typed-ast 1.4.2
typing-extensions 3.7.4.3
urllib3 1.26.5
virtualenv 20.4.7
watchdog 2.1.6
wcwidth 0.2.5
Werkzeug 2.0.1
wheel 0.37.0
```

FFMPEG

Output of `ffmpeg -version`:

```
ffmpeg version 4.4 Copyright (c) 2000-2021 the FFmpeg developers
built with Apple clang version 12.0.0 (clang-1200.0.32.29)
configuration: --prefix=/usr/local/Cellar/ffmpeg/4.4_2 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-avresample --enable-videotoolbox
libavutil 56. 70.100 / 56. 70.100
libavcodec 58.134.100 / 58.134.100
libavformat 58. 76.100 / 58. 76.100
libavdevice 58. 13.100 / 58. 13.100
libavfilter 7.110.100 / 7.110.100
libavresample 4. 0. 0 / 4. 0. 0
libswscale 5. 9.100 / 5. 9.100
libswresample 3. 9.100 / 3. 9.100
libpostproc 55. 9.100 / 55. 9.100
```

Start by running the provided GraphImageBug reproduction with ImageMobject, Graph, and the idle waits, then compare the rendered frames before and after graph animations. Trace the rendering path responsible for draw ordering when a graph is idle; done means the graph's edges remain in front of the image without a no-op animation.

Written by the indexing model from the issue text.