CMake changes for SDL2.
Initial SDL2 support.

This updates it to use the SDL2 APIs, as well as implement a new THRenderTarget,
using SDL_Renderer -- which is HW accelerated.
Clear every frame, make fillBlack = glClear.
Update sdl_audio to SDL2_Mixer.

This also fixes a potential memory leak with rwops.
Simplify - remove other rendering backends.
Remove agg, as we don't use it any more.