xwayland: hook up wl_pointer.axis_v120 events

For details on the protocol itself see the Wayland merge request:

The v120 event has a value base of 120, so one wheel detent is 120, half a
wheel is 60, etc. This is the API Windows has been using since Vista but it
requires HW support from the device. Logitech mice and many Microsoft mice of
the last decade or so have support and it's enabled in the kernel since v5.0.

The new events replace wl_pointer.axis_discrete events, once you bind to
wl_pointer >= 8 you only get the v120 events. So backwards compatibility
is simple, we just multiply the discrete events if we get them and
treat everything as 120 event internally.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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