First cut at SDL2 support. Being kinda problematic.
Get SDL working on Linux.

It turns out that You can no longer include "SDL/whatever.h"
because that is SDL1 headers and thus horribly wrong.
Some cleanup, and experiments for why SDL2_mixer is misbehaving
more cleanups
Merge branch 'master' into SDL2
MinGW support for SDL2 backend
Warn about .WAV format requirements