[ENH]: Add fill between support for 3D plots

[gerazov]

Problem

I couldn't make fill between to work for my 3D plot of a unit circle with a spectrogram sitting on top.

I went through the example on the website (https://matplotlib.org/stable/gallery/mplot3d/polys3d.html) but couldn't adapt it to my plot.

Proposed solution

Finally I found a repo that implements a fill_between_3d function and I could easily make it work.

Please consider adding it to matplotlib.

[scottshambaugh]

This looks very useful, the referenced code is simple, and it's also MIT licensed so could be brought directly in to matplotlib without issues. @artmenlope would you be interested in doing that?

[scottshambaugh]

As a side note @gerazov, do you have a link to the code you used to generate that plot? It's very nice!

[gerazov]

Thanks! It took me a while to make it 😅

If it makes sense I can create a PR for your examples section?