Doc: Add SDL sprite handling code documentation.
Codechange: Introduce THColour decoding functions.
Merge pull request #5 from jorgenpt/sdl2

SDL2: Keyboard fixes
Merge pull request #14 from Alberth289346/sdl2docs

Doc: Add SDL sprite handling code documentation. (Based on Alberth and jorgenpt's approval)
Pass SDL_Renderer* through to movieplayer.
Allow th_movie.cpp to build with SDL2.

This does NOT work - it does not currently convert from FFMPEG frames to SDL2's
SDL_Texture.
Don't return garbage from l_movie_set_renderer.