Merge pull request #23 from jorgenpt/sdl2-refactoring

Some minor cleanup in the SDL GFX backend.
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.
Properly upload and render textures from movies