Added bindings for SDL_GetWindowSurface, SDL_UpdateWindowSurface, SDL_Fi...

...llRect and SDL_MapRGB

I was already playing around with SDL2 and Crystal, then you talked about this project on Crystal's IRC today. Since you were already ahead I figured it'd be more productive to just throw what I had and add to your work.

On this commit, I just added bindings to a couple of SDL2 functions to LibSDL2. Later, I'll try to factor them into the SDL2 module to provide them more idiomatically.