Experimental SDL2 support

Run cmake with the argument "-DUSE_SDL2=true" to enable SDL2
Merge pull request #2 from dotfloat/sdl2

Experimental SDL2 support

TODO: Review GL_SetVideoMode
Accuracy improvments imported from Kex :D

r1275 | svkaiser | 2014-02-25 21:04:12 -0800 (Tue, 25 Feb 2014)

+ New linedef flag (NOMONSTERTRIGGER) discovered. Fixes bug with Imps
activating the secret exit door in Spawned Fear
+ Major code change to collision logic in order to properly emulator key
bump/grabbing
+ Line trigger/shooting behavior more accurate to original
Fix REBORN playerstate being set on password games
Fix erroneous messages on password screen
Renamed flag, fix bug in P_Move(r1276 import)

r1276 | svkaiser | 2014-02-26 18:28:56 -0800 (Wed, 26 Feb 2014)

+ Renamed new flag to ML_HIDEAUTOMAPTRIGGER
+ Fixed bug in P_Move where monster could actually trigger ANY kind of
line special instead of only useable lines
Reimplement password playerstate solution

I didn't find any bugs per se, but Kaiser noted that it was possible
that some variables weren't being properly initialized. Better write a
safe function than figure out the hard way when something blows up.
Ignore warnings about unused parameters

In addition, the dgl.h macros would get an extra semi-colon when expanded.