Add an rcparam for image.interpolation_stage.

[anntzer]

PR summary

As discussed in #18782, where the rcparam was introduced, I would probably always set this to "rgba"...
One question is whether we want to switch back "classic" mode to also use "rgba", which does correspond to pre-2.0 behavior.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[anntzer]

Thanks, both issues fixed.