Fix up SDL2 frontend on windows and clean it up
Merge remote-tracking branch 'origin/sdl2-windows' into elly-bg-rewrite

Conflicts:
	src/frontends/sdl2/audio.c
	src/frontends/sdl2/event.c
	src/frontends/sdl2/input.c
	src/frontends/sdl2/video.c
Merge branch 'elly-bg-rewrite'