wayland/xdg-shell: Add parent window validation in get_popup()

Merged Robert Mader requested to merge rmader/mutter:xdg-popup-parent-window into master

Like in most similar places we can not assume to get a valid window when calling meta_wayland_surface_get_window as it e.g. might be unmapped already.

Calling get_popup on an unmapped window is a client bug, thus post a protocol error when this happens.

Closes #1174 (closed) Closes #1293 (closed)

Merge request reports