When filtering out the events for "window" from the events we got for
our "impl_window", don't forget to adjust the returned number of
events because it might be smaller than what XGetMotionEvents has
returned, and free coords we allocated too much. Also if we filtered
away *all* events, return FALSE and get rid of the allocated history
entirely. Together fixes all sorts of mishehavior when painting in
GIMP, from coords going wild to plain crashes and infinite loops.