Fix SDL2 on Windows

Need to test it on Unix before merging to master
More fixes

#include <SDL.h> because it is needed
Shut up warnings on Unix

Hilariously, even at /W4 MSVC doesn't care about these redefinitions
because they are redefined to the same value so it doesn't matter.
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