sdl2: use ABGR8888 texture on little endian system

This is a massive performance boost on slow processors, because
before, the intermediary buffer had to be swizzled to ABGR8888
before being uploaded -- for large resolutions this was an
enormous performance penalty.
sdl2: I_Error if we can't create gl context
sdl2: software: clear window on mode change
Merge remote-tracking branch 'Furyhunter/sdl2' into sdl2