MNT: Commonize 3D zmargin handling with x and y axes - #31287

I'm pretty sure many of these tests that are changed here have a "Remove this line when the image is regenerated." comment in them that should be heeded.

Good catch, took those off of the 5 that had it. Also took off the tolerance on one, I'll see if it needs to be added back after cross-platform CI.

Edit: looks good

I haven't checked, but I'm not sure if some of these changes will conflict with the text overhaul branch? I guess we need some decision on some of the questions in the original post though.

Some images will need to be regenerated but I don't see the code changes conflicting with text edits.

To summarize, the choice that needs to be made is between these. (1) is what's in right now, but I don't care too much either way:

1. Keep existing 0.05 margins on all axes, understanding that this will "squish" most 3D plot types in Z
2. Move the Z margin to 0 to keep most plot types looking the same (though scatter plots will still change since they were being forced to a minimum of 0.05)

2. Move the Z margin to 0 to keep most plot types looking the same (though scatter plots will still change since they were being forced to a minimum of 0.05)

I have a slight preference for this option to avoid the subtle squishing

Talked about this a bit on the call today, general sentiment was to change default zmargin to 0 to keep most plot types "unsquished" and looking the same as they do now (scatter and logscales would change appearance). And since fixing this means a visual change to something, so we might as well pick the "prettier" option. But some question around if it needs a formal deprecation. @timhoffm thoughts?