No window quads are generated for sub-surfaces right now. This leads to
issues with effects that operate on window quads, e.g. magic lamp and
wobbly windows. Furthermore, the OpenGL scene needs window quads to
properly clip windows during the rendering process.

The best way to render sub-surfaces would be with a little help from a
scene graph. Contrary to GNOME, KDE hasn't developed any scene graph
implementation that we could use in kwin. As a short term solution, this
change adjusts the scene to generate window quads.

Window quads are generated as we traverse the current window pixmap tree
in the depth-first search manner. In order to match a list of quads with
a particular WindowPixmap, we assign an id to each quad.

BUG: 387313
FIXED-IN: 5.19.0

Differential Revision: https://phabricator.kde.org/D29131