Disables input grabbing in Linux - allows alt-tabbing and other actions.

SDL_SetWindowGrab is bugged in Linux.  For some unfathomable reason, possibly a bug all the way upstream in X11, it will always grab the keyboard input.  Grabbing the keyboard input means that alt-tabbing is completely impossible.  This changes around comments in the code so that, when using Linux, input grabbing is avoided while the mouse always enters the window.  An #ifndef is used to ensure that only Linux systems are effected by this change.

While fixing this, I noticed another bug.  For some reason, #ifdef LINUX does not get triggered when compiling for LINUX64.  As such, I placed some extra defines here to solve the issue.  This should really be placed in a better location, or another fix implemented.  As it's virtually required for this fix, unless you like copypasta, I included it directly above the refactored grabbing/warping code in absence of knowing a good place to put it.