wayland/subsurface: Implement meta_wayland_surface_get_window()

Subsurfaces are special regarding windows as they don't have a window, but usually have an ancestor which does. All current users of get_window() are either used for known surface roles, such as xdg-* ones, or, as is the case for pointer constrains, would actually want to get the ancestors window.

Thus implement get_window() to allow pointer constrains to work.

Closes: #2223 (closed)

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This is a new attempt after !2681 (merged). I have tested it for roughly two weeks on my two production machines without running into regressions.