OpenGL wall Y offset

Fixed wall textures breaking when Y offset of the wall was too high.
Merge https://github.com/STJr/SRB2
MD2 Texture load fix

Fixes MD2's using sprites on the first frame when they're loaded.
OpenGL Splitscreen Fix

First screen in split-screen is now not completely black.
OpenGL Screen Fading Fix

Screen Wipes now work as they did in 2.0.x.
OpenGL Translucent Midtexture Fix

Translucent Midtextures using holes with textures in them now work when
there is no FOF's above their sector.