Remove unused bbox arg to _convert_agg_to_wx_bitmap.

[anntzer]

This arg is unused since 744b7f8; we only need to clip when actually
blitting to the destination bitmap, not when first converting the agg
buffer to a bitmap. Also make the conversion function directly take an
rgba buffer as input and rename it, as it isn't really agg-specific.

Also switch the construction of MemoryDC to directly pass in the
Selected bitmap as arg, rather that having a separate SelectObject call
(this is supported all the way back since wx3).

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).