Handle colors for mesh3d in pgfplotsx backend

[darnstrom]

Description

This PR adds support for colors in mesh3d in the pgfplotsx backend.
The example in #5213 now correctly creates a blue triangle (similar to the GR and PlotlyJS backend.)
I have also tested it with several calls to mesh3d, and the result is the same as for GR and PlotlyJS.

Attribution

• I am listed in the appropriate version of .zenodo.json

Things to consider

• Does it work on log scales?
• Does it work in layouts?
• Does it work in recipes?
• Does it work with multiple series in one call?
• PR includes or updates tests?
• PR includes or updates documentation?

[BeastyBlacksmith]

Great, would you mind opening a PR against v2 also?

There the backend code is located here:

Plots.jl/PlotsBase/ext/PGFPlotsXExt.jl

Line 723 in a58d0ff

function pgfx_add_series!(::Val{:mesh3d}, axis, series_opt, series, series_func, opt)