gdk: strengthen touch crossing event synthesizing on programmatical crossings

gdk: strengthen touch crossing event synthesizing on programmatical crossings

There are cases where crossing events aren't generated by input devices themselves
but rather through programmatical means (windows being moved/hidden/destroyed while
the pointer is on top).

Those events come from X as sourceid=deviceid, and GDK does its deal at lessening
this by setting a meaningful source device on such events, although this caused
some confusion on the mechanism to block/synthesize touch crossing events that
could possibly cause bogus enter events on the new window below the pointer.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=691572