Fix for black screen on Linux upon starting a game

Depends on @magcius's branch to be merged.
This won't work correctly until you merge my branch. If you try this in NoGUI, it will look super stretched and bad since in that case GLWin.parent is the root window.

After my branch, GLWin.parent is always the size of the host, which will be correct. We have settings, but as far as I know they don't work correctly for Render to Main Window or NoGUI. If they do, sure, we can use the settings instead. I'm not sure what AGL/WGL/D3D do. If they use the settings, then let's just use those.