VIZ: Fix head size

[drammock]

this makes it so that plot_projs_topomap uses the same sized head for EEG and MEG subplots within a figure (unless user passes their own axes). It's a quick fix to make the docs look nicer before release; was mentioned in #8640

cc @hoechenberger @mmagnuski

Not sure if there are other places in the codebase where we want to do this?

[mmagnuski]

Looks good!

[hoechenberger]

Doc artifact: https://24068-1301584-gh.circle-artifacts.com/0/dev/auto_tutorials/preprocessing/plot_50_artifact_correction_ssp.html#repairing-ecg-artifacts-with-ssp

Looks much better now!

@drammock The "compat" CI run fails:

2020-12-12T00:22:39.5146016Z =================================== FAILURES ===================================
2020-12-12T00:22:39.5146681Z ___________________________ test_plot_projs_topomap ____________________________
2020-12-12T00:22:39.5147493Z mne/viz/tests/test_topomap.py:111: in test_plot_projs_topomap
2020-12-12T00:22:39.5153346Z     plot_projs_topomap(projs, info=info, colorbar=True, **fast_test)
2020-12-12T00:22:39.5154340Z mne/viz/topomap.py:354: in plot_projs_topomap
2020-12-12T00:22:39.5155007Z     ax.sharex(axes[0])
2020-12-12T00:22:39.5156421Z E   AttributeError: 'AxesSubplot' object has no attribute 'sharex'

Do you think you could address the issue with the seemingly cropped figures, too?

[drammock]

@drammock The "compat" CI run fails:

argh, yeah, looks like using sharex after axes creation was only added in MPL 3.3.0. I'll have to see if there's a way to do it at creation time that doesn't break other plot types (it uses shared _prepare_trellis code).

Do you think you could address the issue with the seemingly cropped figures, too?

I haven't yet had a chance to dig into why this is happening in sphinx but not in interactive plotting (probably a backend difference? I've at least ruled out the idea that sphinx-gallery was secretly calling tight_layout...). Separate PR, but sadly I'm off for the next 5 days or so, so might not make the 0.22 release unless someone else looks into it.