Failed to set the window size in window.py

### Describe the bug
Manim runs with default window size (1280, 720) instead of default value described in config.py (half the screen size).

**Code**:
add following lines to the `__init__` method of class `Window` in `window.py`:
```python
self.size = size
+ print(size)
+ print(self.size)
```
**Wrong display or Error traceback**:
terminal output:
```
(683, 384)
(1280, 720)
```

### Additional context
OS: arch linux, gnome desktop environment.