wayland: Ensure constraints after client resize

This first fixes an oversight in our client resize detection which is needed for the test to work, then adds queuing a mutter-side resize after a client resize to enforce constraints and finally adds a test.

I expect that the gboolean has_pending_size(window) introduced here might need to be changed to a gboolean get_pending_resize(window, *width, *height) soon for fixing #1627. Should I do that in this MR already or only when it is actually needed?