screen-cast: Let the reason for recording determine what to record

E.g. we'll have pointer movement that, if no painting is already
scheduled, should only send new cursor metadata without any new pixel
buffer. When this happens, tell next step to not record the pixels if
this was the case, instead of having it rediscover this itself.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1323
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1351