windowManager: Discard workspace scroll smooth events

windowManager: Discard workspace scroll smooth events

Since touchpad smooth scroll events with source finger are handled by
the swipeTracker, the workspace scroll handler can focus on discrete
events.

Thanks to Mutter emulating discrete scroll events, see
meta_seat_impl_notify_scroll_continuous_in_impl in meta-seat-impl.c,
it is safe to ignore smooth scroll in the workspace scroll handler
and handle exclusively discrete events.

In addition, once high-resolution scroll events land in Mutter [1], a
mouse will be able generate non emulated smooth scroll events that
should be ignored in favour of the discrete scroll events.
Otherwise, a single mouse wheel click will scroll through multiple
workspaces at once.

[1] https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1962

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1959>