Add support for RGB and RGBA colors to pcolormesh

[kylehofmann]

Rationale

Matplotlib's Axes.pcolormesh allows users to specify three types of data: Scalar data (used together with a color map), RGB data, and RGBA data. Prior to this patch, GeoAxes.pcolormesh only supported scalar data. This patch adds support and tests for RGB and RGBA data in GeoAxes.pcolormesh.

Implications

Additional visualization options. I made https://www.reddit.com/r/dataisbeautiful/comments/13vq5pg/oc_earths_magnetic_field/ this way.

Improved compatibility with Axes.pcolormesh. Compatibility is better but not perfect; wrapping RGB or RGBA data is not supported because GeoAxes.pcolormesh ultimately calls Axes.pcolor, which does not support RGB or RGBA data.

[rcomer]

Thank you for your contribution @kylehofmann - this seems to be a feature that a lot of people want. See also #2166, which should support the wrapping but is waiting for matplotlib/matplotlib#25027 which will enable pcolor to handle RGB(A).

[kylehofmann]

@rcomer I wish I had thought to look for something like #2166! It's certainly more thorough than this patch. I particularly like how you handled the tests! (Though if I could make a suggestion: Instead of having PARAMETRIZE_PCOLORMESH_WRAP be True or False, make it standard, rgb, or rgba. When it's one of the latter two, get the mapped data as you do now; then, if it's rgb, set data = data[..., 0:3].) Given that #2166 exists, there doesn't seem to be any reason to accept this PR. Thanks for your time!

[rcomer]

Thanks @kylehofmann. Please feel free to add any suggestions/reviews to #2166.