The idea is just to pass self.dpi to savefig() and then use
self.frame_size as the frame size rather than looking up renderer
internals, but this is made easier by properly moving some attributes
and methods (some of__init__,setup,frame_size) from MovieWriter
(which should really be called SubprocessMovieWriter, as that's what
it does) to AbstractMovieWriter, so that PillowWriter can reuse
them without having to carefully prevent any attempt of starting a
subprocess.

Closes#15678, attn@dopplershift.

I... don't understand the doc build failure, likely something with autosummary and inheritance?
Edit: looks like that's indeed the problem, there's already a missing-references entry for PillowWriter.frame_size so I don't feel too bad adding one for MovieWriter.frame_size (now that it's defined in AbstractMovieWriter.frame_size).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code isFlake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way