clutter/paint-context: Add paint flag

A paint flag affects a paint operation in ways defined by the flags.
Currently no flags are defined, so no semantical changes are defined
yet. Eventually a flag aiming to avoid painting of cursors is going to
be added, so that screen cast streams can decide whether to include a
cursor or not.

Changes for gnome-3-36: Removed flag from offscreen context.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1222