Many references to the dixScreenOrigins array already had the
corresponding screen pointer handy, which meant they usually looked like
"dixScreenOrigins[pScreen->myNum]". Adding a field to ScreenRec instead
of keeping this information in a parallel array simplifies those
expressions, and eliminates a MAXSCREENS-sized array.

Since dix declared the dixScreenOrigins array, I figure allocating a
screen private for these values is overkill.

Signed-off-by: Jamey Sharp <jamey@minilop.net>
Reviewed-by: Tiago Vignatti <tiago.vignatti@nokia.com>
Tested-by: Tiago Vignatti <tiago.vignatti@nokia.com> (i686 GNU/Linux)