Main motivation is from T54314 where there was no way to read from a
Viewer image datablock after the compositor has run.
The only solution there was to do a full rerender (which obviously takes
much longer). Adding a handler avoids having to rerender.
This uses new syntax from rBf4456a4d3c97 and also adds "COMPOSITE" as a
job type that can be queried by `bpy.app.is_job_running`.
NOTE: there is another issue when multiple viewers are used and these
get active via RNA (compo execution is not triggered there yet -- unlike
when a viewer is selected in the Editor -- this is an issue of
`ED_node_set_active` vs. only `nodeSetActive`, but this will be tackled
separately)
Maniphest Tasks: T54314
Differential Revision: https://developer.blender.org/D15078
16d329da28