Generate summary when (only) --mpl-generate-summary passed

[kavanase]

Hi!
This is an incredibly useful package; we use it extensively in our tests for doped and ShakeNBreak; so thank you very much for building and maintaining!

Just a question/suggestion; when running pytest --mpl-generate-summary=html no summary file is created, but it is with pytest --mpl --mpl-generate-summary=html, because the catch here (https://github.com/matplotlib/pytest-mpl/blob/29fd89095ced454e1f82007aeb592268a430aba0/pytest_mpl/plugin.py#L226C1-L230C7) accounts for --mpl, --mpl-generate-path and --mpl-generate-hash-library. As this is one of the commonly used options, maybe it would be good for it to have similar behaviour to --mpl-generate-path, not needing --mpl to also be specified?
(Obvs a 'me' problem, but personally I always forget I also need --mpl when trying to use the summary option, as I don't use it when generating the images --mpl-generate-path, and takes me a little while to figure out why it's apparently not working).

Maybe something like checking if any "mpl-generate" option has been specified could be used here?