enable depth peeling by default

[banesullivan]

Resolve #527 by controlling depth peeling (#450) in the rcParams. Default is now enabled.

[akaszynski]

This will improve rendering for some of the models I use my closed source project. I agree with adding to rcParams. Thanks for adding that to the documentation.

[banesullivan]

@GuillaumeFavelier, are you cool with this? I imagine it will only benefit mne-python

[GuillaumeFavelier]

Yes, I'm okay with it!

Depending on user feedback I can always change it locally and/or report in here 👍

[GuillaumeFavelier]

For now I have to disable the depth peeling in MNE, I noticed some artifacts in a use case with overlapping surfaces:

With depth peeling	Without depth peeling

I tried different parameters (increasing the number of peels and decreasing the occlusion ratio) but I got no improvement.

To reproduce on mne-python/master:

$ MNE_3D_BACKEND=pyvista ipython --gui=qt
In [1]: run tutorials/source-modeling/plot_visualize_stc.py

I have to create a minimal working example using pyvista only to help isolate the issue but it's totally possible that there is something wrong in my code.

[banesullivan]

Perhaps messing with the coincident topology options would help with that? Some details in #301 (comment)

@GuillaumeFavelier, can you save out those two surfaces as VTK files and post them?