Deprecate dummy renderers (which are only used with dryrun=True, which
is itself deprecated).

Move the image-to-stream warning to draw_image.

Deprecate GraphicsContextPgf (we can just use GraphicsContextBase
directly; e.g. the Agg backend does the same).

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