windowManager: Warn when removing old animationInfo

windowManager: Warn when removing old animationInfo

Now that we properly notify mutter about when a size-change animation
has ended, it should never happen that a new size-change animation is
started without the last one being cancelled (ie. 'kill-window-effects'
being emitted).

This means there should also never be an old animationInfo attached to a
window actor, so warn in case we still find one when starting the
animation.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1251