When calculating the resource scale of a clone source, we might end up
in situations where we fail to do so, even though we're in a paint. A
real world example when this may happen if this happens:
* A client creates a toplevel window
* A client creates a modal dialog for said toplevel window
* Said client commits a buffer to the modal before the toplevel
If GNOME Shell is in overview mode, the window group is hidden, and the
toplevel window actor is hidden. When the clone tries to paint, it fails
to calculate the resource scale, as the parent of the parent (window
group) is not currently mapped. It would have succeeded if only the
clone source was unmapped, as it deals with the unmapped actor painting
by setting intermediate state while painting, but this does not work
when the *parent* of the source is unmapped as well.
Fix this by inheriting the unmapped clone paint even when calculating
the resource scale.
This also adds a test case that mimics the sequence of events otherwise
triggered by a client. We can't add a Wayland client to test this, where
we actually crash is in the offscreen redirect effect used by the window
dimming feature in GNOME Shell.
Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/808https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1147
32c99513c8