Disables input grabbing in Linux; allows alt-tabbing and other actions.
Fix the previous commit to work on Linux32.  Add more comments.
Adds command line parameter -mousewarp to control the use of warping in place of grabbing.  Works around bugs in certain versions of X.
Merge remote-tracking branch 'ilag11111/sdl2' into sdl2

Conflicts:
	src/sdl2/i_video.c