[platforms/x11] Get rid of some code duplication in windowed backend

Summary:
We repeat quite a lot of code that finds an output by xcb_window_t and
translates global X11 screen coordinates to output coordinates.

Test Plan: Compiles.

Reviewers: #kwin, gladhorn

Reviewed By: #kwin, gladhorn

Subscribers: gladhorn, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23947