Add Xinerama support for GHOST_GetMainDisplayDimensions() so X11 works as it should (previously would include all monitors).

Now the active monitor size is used on startup. 
Currently the cursor position is checked for intersection with the monitor bounds to find the active screen.



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54227 954f8c5b-7b00-dc11-b283-0030488c597c