fix default image format in gtk4 savefig dialog

[ahaldane]

PR summary

When I try to save a figure using the gtk4 file chooser the default format is PGF code for LaTeX, but it should be "png" because I set rcParams["savefig.format"] = 'png' . The default file extension is still .png , confusingly. This only started happening after I recently upgraded my system to gtk4.

Looking at the code, I see an indexing bug when choosing the default format: The previous lines to my diff reorganized the format order so that the default format is the first in the list, therefore to get the default one should use index 0, not the old index before reordering.

PR checklist

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

I manually tested that the dialog works as expected after this PR. I did not run the unit tests, nor create a unit test, but I can run them if requested. I hope that CI can be enough for such a small change as I haven't set up a dev env for matplotlib.

[dstansby]

Good catch! I presume there isn't an easy way to add a test for this?

[QuLogic]

Unfortunately, there is no way to automate GUI tests like these.