Adds basic support for the "wheel" event from the Wayland tablet protocol.
Ideally we would accumulate the angle and report a wheel event with an
appropriate value for "clicks". We can get away with a much cruder method
for the time being, however, since no Wacom tablet puck actually provides
a smooth scrollwheel. Checking whether the angle in CLUTTER_INPUT_AXIS_WHEEL
exceeds a nominally-small threshold is sufficient to determine that the
wheel has advanced by at least one physical click.
https://bugzilla.gnome.org/show_bug.cgi?id=783716
4d8cb5408b