Support masked array inputs for to_rgba and to_rgba_array.

[efiring]

PR Summary

Closes #14301.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 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

[efiring]

The Travis failure is an unrelated glitch in the 3.7 build: a failure to add a ppa for ffmpeg.

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.1.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 4a0d9c51a3794e99ab802b6fcbab591dffa1f440

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #14313: Support masked array inputs for to_rgba and to_rgba_array.'

4. Push to a named branch :

git push YOURFORK v3.1.x:auto-backport-of-pr-14313-on-v3.1.x

5. Create a PR against branch v3.1.x, I would have named this PR:

"Backport PR #14313 on branch v3.1.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.