First attempt at SDL2 port -- buggy
Continue SDL2 migration
Merge branch 'master' into sdl2
Merge branch 'master' into sdl2

Conflicts:
	Makefile
Merge branch 'master' into sdl2
Merge branch 'master' into sdl2
Merge branch 'master' into sdl2

Conflicts:
	callbacks.cc
	controls.cc
	sdl_main.cc
Handle mouse wheel events in the SDL 2 way
Handle window resize events in the SDL 2 way
Add comment about receiving no-op resize events
Make mouse drag handling SDL2-compliant
Add partly-functional SDL2 keyboard events
Move SDL-AntTweakBar interface into its own file, and start converting
AntTweakBar's SDL 1.3 interface to work with SDL 2.
Cleanup
Handle ATB mouse wheel events correctly
Handle ATB keyboard events properly.
It's not clear if the modified AntTweakBar code is entirely correct,
but it appears to be good enough for our purposes.