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

Summary:
I noticed that while working on my GSoC Xwayland project and using sub-surfaces with empty input region. 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.

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