Scanouts are taken away after painting. However, when we're
streaming, what we actually want is to capture whatever is
going to end up on screen - and that includes the scanout
if there's any.
Add a before-paint watch that only records new frames if a
scanout is set.
Inspired by (and commit log mostly copied from) e6a13e5d57
("monitor-stream-src: Add before-paint watch to catch scanouts").
v2:
* Do not call stage_painted from before_stage_painted (Georges Basile
Stavracas Neto)
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1707
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1902>