write_to_movie option failed in manim.cfg

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## Description of bug / unexpected behavior

`write_to_movie` option in manim.cfg cannot be detected. but the command line `--write_to_movie` works

## Expected behavior

if we write `write_to_movie = True` in manim.cfg and use it as config file, then we don't need to use `--write_to_movie`

## How to reproduce the issue

run these two comman line:
```
manim main.py -c manim.cfg
manim main.py -c manim.cfg --write_to_movie
```

main.py

```py
from manim import *
class DefaultTemplate(Scene):
def construct(self):
circle = Circle() # create a circle
circle.set_fill(PINK, opacity=0.5) # set color and transparency

square = Square() # create a square
square.flip(RIGHT) # flip horizontally
square.rotate(-3 * TAU / 8) # rotate a certain amount

self.play(Create(square)) # animate the creation of the square
self.play(Transform(square, circle)) # interpolate the square into the circle
self.play(FadeOut(square)) # fade out animation
```

manim.cfg

```
renderer = opengl
write_to_movie = True
preview = True
```

## Additional media files

## Logs

## System specifications

System Details

- OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)): Windows 11
- RAM:
- Python version (`python/py/python3 --version`):Python 3.8.12
- Installed modules (provide output from `pip list`):
```
Package Version Location
------------------------------ --------- ----------------------------------------
azure-cognitiveservices-speech 1.22.0
backcall 0.2.0
certifi 2021.10.8
cffi 1.15.0
charset-normalizer 2.0.7
click 8.0.3
click-default-group 1.2.2
cloup 0.13.1
colorama 0.4.4
colour 0.1.5
commonmark 0.9.1
comtypes 1.1.11
debugpy 1.5.1
decorator 5.1.0
entrypoints 0.3
glcontext 2.3.4
gTTS 2.2.4
idna 3.3
ipdb 0.13.9
ipykernel 6.4.2
ipython 7.29.0
ipython-genutils 0.2.0
isosurfaces 0.1.0
jedi 0.18.0
jupyter-client 7.0.6
jupyter-core 4.9.1
manim 0.15.2
manim-fontawesome 6.0.0
manim-physics 0.2.4
manim-speech 0.0.1 e:\manim视频\测试-speech插件\manim-speech-main
ManimPango 0.4.1
mapbox-earcut 0.12.10
matplotlib-inline 0.1.3
moderngl 5.6.4
moderngl-window 2.4.0
multipledispatch 0.6.0
mutagen 1.45.1
nest-asyncio 1.5.1
networkx 2.6.3
numpy 1.21.3
parso 0.8.2
pickleshare 0.7.5
Pillow 8.4.0
pip 21.0.1
prompt-toolkit 3.0.21
pycairo 1.20.1
pycparser 2.21
pydub 0.25.1
pyglet 1.5.21
Pygments 2.10.0
pymunk 6.2.1
PyOpenGL 3.1.6
pypiwin32 223
pyrr 0.10.3
python-dateutil 2.8.2
python-dotenv 0.20.0
pyttsx3 2.90
pywin32 302
pyzmq 22.3.0
requests 2.26.0
rich 10.12.0
scipy 1.8.1
screeninfo 0.8
setuptools 58.0.4
Shapely 1.8.2
six 1.16.0
skia-pathops 0.7.1
sox 1.4.1
srt 3.5.2
toml 0.10.2
tornado 6.1
tqdm 4.62.3
traitlets 5.1.1
typing-extensions 4.2.0
urllib3 1.26.7
watchdog 2.1.6
wcwidth 0.2.5
wheel 0.37.0
wincertstore 0.2
```

## Additional comments

Start by running the two commands in the issue with main.py and manim.cfg, comparing the behavior with and without --write_to_movie. Trace how the -c manim.cfg option is read and how write_to_movie is handled, then verify that the configuration alone enables movie output while preserving the other settings.

Written by the indexing model from the issue text.