Update to latest SDL2

[leo60228]

A merge is non-trivial since SDL2 upstream is now on GitHub instead of using Mercurial. However, I'm not clear how important this is. SDL2 upstream works fine, though #define HAVE_FMOD 1 and building events/SDL_displayevents.c are necessary.
I worked around this on the SDL2 side, which let me port VVVVVV (a project that requires latest SDL2 to build): leo60228/SDL@579384c