Fix OpenGL Fademasks (Screen Transitions)

Screen transitions don't work in OpenGL when scr_depth is set to 8 or 16 bits. This fixes that.

tested it out myself earlier. All seems good, fades definitely work again :)

Can you push this branch to your github repos, so I can have the buildbot test it out?

https://github.com/Sryder13/SRB2/commits/gl-fademask-fix I think this should be right, let me know if it's not.

OGL plays fades upside down.

Test case: https://dl.dropboxusercontent.com/u/3518218/21/xsrb2fade.wad

Added 1 new commit:

• 0eb41b44 - Flip fade mask Y coordinates

Fades should be right way up now.

Status changed to merged

mentioned in commit a55f2077