gl: abstract the GL implementation
SDL: disable timidity build in SDL_mixer
gles: mask out unsupported calls in GLES
screen: remove unused method
gl: privately import EGL and extensions
SDLActivity: sync with upstream
android: patch SDL for a missing symbol
gl: move to a 'support' library

Other things should go here (such as Android path differences).
android: factor out data and asset paths
android: initialize the garbage collector