ENH: Add volumetric rendering to _Brain

[larsoner]

It would be nice to make MixedSourceEstimates and VolSourceEstimates plottable with _Brain instead of just surfaces.

Random ideas:

• Slicer is probably a good resource for ideas, and context preserving looks cool

• VTK example produces:

  

• A random ray-tracing lib, MIT license

• A recent issue about it

• 5-year-old post from VTK folks

• 5-year-old post, using Python

Alternatively we could go with isocontours, but seems harder to control/configure

[agramfort]

#nerdsniping on the way...

…

[GuillaumeFavelier]

Another useful source?

https://docs.pyvista.org/examples/02-plot/volume.html?highlight=volume

[larsoner]

Yes much more useful!