MetaGestureTracker has been separating the "did I handle an event?" and the
"should the event be filtered out?" questions, merge this and make
handle_event() reply to "should the event be only handled by me?".
If a sequence wasn't accepted yet by the gesture tracker, the event will
go through (eg. not handled exclusively by the gesture tracker) and it'll
still be processed by Clutter, triggering gesture actions, and maybe
changing the sequence into other state.
https://bugzilla.gnome.org/show_bug.cgi?id=733631
Due to the way the MetaGestureTracker processes every touch event, this
will tell as closely to Clutter as possible the current number of touches
happening on the stage.
Even though, this is subject to windowing behavior, on X11, rejected touches
will be soon followed by a XI_TouchEnd event, so the compositor will stop
seeing touch sequences that are still operating on clients. On wayland, touch
sequences are processed by the compositor during all their lifetime, so these
will stay on the MetaGestureTracker with META_SEQUENCE_PENDING_END state, yet
still tracked.
https://bugzilla.gnome.org/show_bug.cgi?id=733631
