Resolves the issue reported in #120587, which occurred when bumping the
supported version of the seat API to 9.
Smooth scrolling values were set in without calling the discrete step
callback. Resolve by calculating discrete values from the:
wl_pointer_listener::axis_value120 handler.
The functional changes from this commit only apply once the version
of the seat API is increased - which isn't part of this commit.
df87770ca1