Many references to the WindowTable array already had the corresponding
screen pointer handy, which meant they usually looked like
"WindowTable[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 uses this data, a screen private entry isn't appropriate.

xf86-video-dummy currently uses WindowTable, so it needs to be updated
to reflect this change.

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)