OpenGL HUD scaling fix

This fixes OpenGL's HUD not placing the Score/Time/Rings stats, as well as other HUD items such as the Race countdown numbers + GO graphics, in the right places on screen in non-green resolutions, compared to Software mode.

Have some images for comparison (both taken in 640x480):

Before fix:

After fix:

I also probably fixed an issue with HUD items using V_SNAPTO* flags combined with the scaling flags, but I haven't tested this out properly yet.