Frame events group logically connected pointer events. It makes sense to make
the backend responsible for sending frame events, since once the events are
split (ie. once the frame events are stripped) it's not easy to figure out
which events belongs to which frame again.
This is also how Weston handles frame events.
Fixes https://github.com/swaywm/wlroots/issues/1468
Whenever a new surface is created, we have to update the cursor focus,
even if there's no input event. So, we generate one motion event, and
reuse the code to update the proper cursor focus. We need to do this
for all surface roles - toplevels, popups, subsurfaces.
Fixes#1162