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
Proof of concept fix for Sonic not being MD2-able
Merge branch 'master' of https://github.com/ilag11111/SRB2

With whitespace fixup
Initial SDL2 renderer impmlementation work.
Stub out unfinished implementations
i_video.c: remove old cruft and dead code
remove hi-res filters
sdl2: remove unsupported platform-specific folders
sdl2: fix memory detection on 64-bit linux
sdl2: it works
sdl2: restore keyboard input
sdl2: support all windowed mode resolutions
sdl2: support windowed fullscreen as default
sdl2: aspect correction in fullscreen, clear on blit
sdl2: fix mode changing