xwayland: Separate DamagePtr into separate window data

This will be dissociated in future commits to handle the cases
where windows are being realized before there is a compositor
handling redirection.

In that case, we still want the DamagePtr to be registered upfront
on RealizeWindowProc before a corresponding xwl_window might be
created. Most notably, it cannot be lazily created on
SetWindowPixmapProc as damage accounting gets broken.

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
(cherry picked from commit 4e50440a)