Summary:
So far KWin didn't send axis_source, axis_discrete, and axis_stop. Even
though most of those events are optional, clients need them to work as
expected. For example, one needs axis_source and axis_stop to implement
kinetic scrolling; Xwayland needs axis_discrete to prevent multiple
scroll events when the compositor sends axis deltas greater than 10, etc.

BUG: 404152
FIXED-IN: 5.17.0

Test Plan:
* Content of a webpage in Firefox is moved by one line per each mouse
  wheel "click";
* Scrolled gedit using 2 fingers on GNOME Shell, sway, and KDE Plasma;
  in all three cases wayland debug looked the same (except diagonal scroll
  motions).

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19000