Paint black bars in OpenGL
The OpenGL video output needs to command drawing of the side black bars, when video and window aspect ratio don't match, or when zoom is forced.
There are several reasons why:
- On Wayland, the window size is intrinsically the content size (see also #18045).
- On X11 (and probably Wayland too), some graphic drivers exhibit much better performance when drawing into a fullscreen surface (see also #7702 (closed)).
- This will eventually be required to render SPU outside the video area.