VgaSDL: Set the window size at runtime.

If the height of the primary display resolution is less than 1024, the size of the window will be set to 800x600; otherwise, it will be set to 1024x768.

I did not make it an error if the resolution height is less than 600. The game creates a window first, and then changes to fullscreen mode. That means it won't be possible to play in fullscreen mode at lower resolutions if the game disallows heights < 600.