This prevents multiple scroll events happening for wayland compositors
which send axis values other than 10. For example, libinput will
typically return 15 for each scroll wheel step, and if a wayland
compositor sends those to xwayland without normalising them, 2 scroll
wheel steps will end up as 3 xorg scroll events. By listening for the
discrete_axis event, this will now correctly send only 2 xorg scroll
events.

The wayland protocol gurantees that there will always be an axis event
following an axis_discrete event. However, it does not gurantee that
other events (including other axis_discrete+axis pairs) will not happen
in between them. So we must keep a list of outstanding axis_discrete
events.

Signed-off-by: Scott Anderson <scott@anderso.nz>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit cd285922)