`Variable` class does not get fixed in the in `ThreeDScene` with `add_fixed_in_frame_mobjects()`

## Description of bug / unexpected behavior
`self.add_fixed_in_frame_mobjects(x)` should fix the object `x` on the 2D screen while one works in a 3D Scene. In fact, it works when `x` is a `Text` class, as shown in the [examples](https://docs.manim.community/en/stable/examples.html#fixedinframemobjecttest). But when `x` is a `Variable` class, its position changes when we perform the animation `self.play(x.tracker.animate.set_value())`

## Expected behavior
`x` should remain on its place as we change the tracker value.

## How to reproduce the issue

Code for reproducing the problem

```py
from manim import *

class FixedInFrameMObjectTest(ThreeDScene):
def construct(self):
axes = ThreeDAxes()
self.set_camera_orientation(phi=75 * DEGREES, theta=-45 * DEGREES)
self.play(Create(axes))
self.wait()
text3d = Variable(0.3, MathTex("x"))
text3d.to_corner(UL)
self.add_fixed_in_frame_mobjects(text3d)
self.play(Write(text3d))
self.wait()
self.play(text3d.tracker.animate.set_value(1))
self.wait()
```

## Additional media files

Images/GIFs


## Logs
Terminal output

```
❯ manim -ql -p test.py
Manim Community v0.16.0.post0

[09/06/22 12:21:48] INFO Animation 0 : Using cached data (hash : cairo_renderer.py:75
166007114_155735041_223132457)
[09/06/22 12:21:49] INFO Animation 1 : Using cached data (hash : cairo_renderer.py:75
723241031_3315967912_2417228262)
[09/06/22 12:21:50] INFO Animation 2 : Using cached data (hash : cairo_renderer.py:75
723241031_1815128715_1682129457)
[09/06/22 12:21:51] INFO Animation 3 : Using cached data (hash : cairo_renderer.py:75
723241031_3315967912_3582625705)
[09/06/22 12:21:52] INFO Animation 4 : Using cached data (hash : cairo_renderer.py:75
723241031_3076078580_3009176059)
[09/06/22 12:21:53] INFO Animation 5 : Using cached data (hash : cairo_renderer.py:75
723241031_3315967912_4108828480)
INFO Combining to Movie file. scene_file_writer.py:607
[09/06/22 12:21:54] INFO scene_file_writer.py:728
File ready at
'/home/tapyu/git/courses/phd_nonlinear_opt/
Exercises/media/videos/test/480p15/FixedInF
rameMObjectTest_ManimCE_v0.16.0.post0.gif'

INFO Rendered FixedInFrameMObjectTest scene.py:240
Played 6 animations
INFO Previewed File at: file_ops.py:224
'/home/tapyu/git/courses/phd_nonlinear_opt/Exercises
/media/videos/test/480p15/FixedInFrameMObjectTest_Ma
nimCE_v0.16.0.post0.gif'
```

## System specifications

System Details

```
OS: Ubuntu 22.04 jammy
Kernel: x86_64 Linux 5.15.0-47-generic
Uptime: 1d 1h 37m
Packages: 2157
Shell: zsh 5.8.1
Resolution: 1366x768
DE: GNOME 41.7
WM: Mutter
WM Theme:
GTK Theme: Yaru-dark [GTK2/3]
Icon Theme: Yaru
Font: Ubuntu 11
Disk: 71G / 469G (16%)
CPU: Intel Core i5-8250U @ 8x 3.4GHz [49.0°C]
GPU: Intel Corporation UHD Graphics 620 (rev 07)
RAM: 1682MiB / 2288MiB
```

LaTeX details

```
❯ tex --version
TeX 3.141592653 (TeX Live 2022/dev/Debian)
```

FFMPEG

```
❯ ffmpeg -version
ffmpeg version 4.4.2-0ubuntu0.22.04.1 Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 11 (Ubuntu 11.2.0-19ubuntu1)
configuration: --prefix=/usr --extra-version=0ubuntu0.22.04.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librabbitmq --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-pocketsphinx --enable-librsvg --enable-libmfx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
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
libswscale 5. 9.100 / 5. 9.100
libswresample 3. 9.100 / 3. 9.100
libpostproc 55. 9.100 / 55. 9.100
```

## Additional comments
I haven't used `Manim` for a while (2 or 3 years), and the syntax change considerably. So my code may have some errors.

Start by running the provided FixedInFrameMObjectTest reproduction with Variable, ThreeDScene, add_fixed_in_frame_mobjects(), and tracker.animate.set_value(). Read the Variable and ThreeDScene behavior involved in fixed-in-frame objects; done means the Variable remains in the same 2D-screen position while its tracker value changes.

Written by the indexing model from the issue text.