Default image extension to .tga

Fixes loading of textures with dots in filename
Use doubles where time to FP conversion is involved
Pass identity origin and angles to GLSL for non-model entities
added SDL2 2.0.3 source code
SDL2 framework for OS X
removed old SDL.framework
FindSDL2.cmake
sdl2
sdl2
sdl2
files from SDL2_ultra_old branch
sdl2
comment out old code
comment out old code
comment out old code
SDL2