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