xwayland: use wayland axis_discrete event

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>