Perf: re-enable VOSF on Xcode-made, OSX-host builds

commented on use of SDL_SetRelativeMouseMode

made windowed-mode windows be resize-able/scale-able - guest OS displ…

…ay size is retained, though

fixed issue zydeco#1: SDL2 backend did not support 2 or 4 bit color, …

…in guest OS

Prerelease 3

bug-fix, Issue cebix#2: prevent SDL2 from installing keyboard shortcu…

…ts (Command+Q, etc.) on OSX hosts, as they interfere with guest OS use

dropped Mac OS X deployment target down to 10.7 (from 10.12)