For two ScreenRecs abs pointer positioning was working fine, but touch events
stuck to the lower/right edge on any screen but the one with a 0/0 origin.
Cause is a missing offset by the screen coordinates, causing the root
coordinates in the event to desktop-wide, not screen-wide.
Offset properly, just like we do for pointer events.
X.Org Bug 86655 <http://bugs.freedesktop.org/show_bug.cgi?id=86655>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>