[wayland] Recursively destroy WindowPixmap objects

Summary:
We need to destroy the root WindowPixmap together with all of its
children; otherwise, buffers that are attached to subsurfaces will
not be released.

Test Plan:
weston-subsurfaces doesn't quit with an error message saying that
all buffers are held by the compositor.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28145