Right now the stage only had a signal called 'after-paint' which was not
tied to painting but updating. Change this to offer 4 signals, for the 4
different stages:
* before-update - emitted in the beginning before the actual stage
updating
* before-paint - emitted before painting if there will be any stage
painting
* after-paint - emitted after painting if there was any stage painting
* after-update - emitted as a last step of updating, no matter whether
there were any painting or not
Currently there were only one listener, that should only really have
been called if there was any painting, so no changes to listeners are
needed.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1285
2afe3e9223