This fixes the following
1. Minimize window; minimize animation starts
2. Do something that immediately destroys the animated actor (e.g. terminate)
3. This triggered the timeline of the animation to emit a "stopped"
signal while all transitions of the actor were destroyed
Previously we'd implicitly animate the scale again (set_scale(..)) which
created a new transition The hash table iterator didn't like this and
abort():ed
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3213>
Except for the tests that launches `mutter`, use a custom shell
implementation. It's roughly a copy of default.c with some cleanups on
top. A custom shell allows for a bit more freedom when doing testy
things.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3185>
