OpenGL Flipped Patch Drawing

V_FLIP in OpenGL.
LightLevelToLum gone

LightLevelToLum unused by everything now, it seemed to be giving wrong
values for lightlevels.
Negative Light

Light no longer goes wrong when lightlevel of a sector is negative.
Redwall sky gone

More redwall sky shenanigans gone.
OpenGL Patch Fill Fix

Patch Fill Drawing for intermission has been fixed to loop properly in
non aspect correct resolutions.
SRB2 2.1.6 release
2.1.6 release

Merge branch 'master' of github.com:STJr/SRB2 into sdl2