It became clear we need a RC3, mainly for 3 reasons (ordered with first
ones being more serious issues) for which I think more testing would be
better:

1. The changes in the image graph, regarding filters (#12614), and
   possibly more than I am thinking of doing before RC3.
2. The change in multi-threading code for paint (#11389). I want to be
   sure not to create new crashes (multi-threading is always full of
   surprises!).
3. The change to a central plug-in API implementation (!2058) and I want
   to make sure it doesn't bite us back with more testing (though it is
   only for persistent plug-ins so it is fortunately not touching every
   plug-in).