OSX: don't avoid unsync mapping on nvida gpus just because the windows driver doesn't like it

OSX has their own driver, so performance issues aren't shared with the nvidia driver (unlike the closed source linux and windows nvidia driver). So now they'll also use the MapAndSync backend like all other osx drivers.

fixes issue 6596

I've also cleaned up the if/else block selecting the best backend a bit.
BBox: Changed the rounding again, the old one fixed Paper Mario but had bugs in Mickey's Magical Mirror. This change fixes the glitches in both games.

Also fixed some compiler warnings.