added definitions for SDL_GetRGB, SDL_GetRGBA, and SDL_putenv 

I added support for 3 new SDL functions which are declared in header files shipped with Emscripten but were not defined:
1) SDL_GetRGB declared in SDL_pixels.h (allows R/G/B component extraction from a pixel value)
2) SDL_GetRGBA declared in SDL_pixels.h (same as above, but with an extra alpha component)
3) SDL_putenv declared in SDL_compat.h (a lot of SDL 1.2 based projects typically use this to center windows)