[server] Respect input region of sub-surfaces on pointer surface focus

Summary:
KWayland takes always the top-most child surface at a given position for its
pointer input. But if a sub-surface sets its input region, it should not select
this one when the position is out of its input region, but rather try the
surface below.

Test Plan:
My testing was only on my Xwayland branch. Supposed to also fix a problem
with Firefox native Wayland port.

Reviewers: Frameworks, graesslin, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, zzag, kde-frameworks-devel, graesslin, plasma-devel

Tags: Frameworks, Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D7038