[BUG] `mne.viz.Brain` volume rendering doesn't show up

If you don't set alpha=0, you get an opaque inflated brain along with your volume source estimate. I think the default arguments could handle this better.

To replicate:

import mne
from mne.datasets import sample
from mne import read_evokeds
from mne.minimum_norm import apply_inverse, read_inverse_operator

data_path = sample.data_path()
meg_path = data_path / 'MEG' / 'sample'
fname_inv = meg_path / 'sample_audvis-meg-vol-7-meg-inv.fif'
fname_evoked = meg_path / 'sample_audvis-ave.fif'

snr = 3.0
lambda2 = 1.0 / snr ** 2
method = "dSPM"  # use dSPM method (could also be MNE or sLORETA)

# Load data
evoked = read_evokeds(fname_evoked, condition=0, baseline=(None, 0))
inverse_operator = read_inverse_operator(fname_inv)
src = inverse_operator['src']

# Compute inverse solution
stc = apply_inverse(evoked, inverse_operator, lambda2, method)
stc.crop(0.0, 0.2)

brain = mne.viz.plot_source_estimates(stc, subject='sample', subjects_dir=data_path / 'subjects', src=src)

Start by running the provided sample-data reproduction and inspect the mne.viz.Brain and mne.viz.plot_source_estimates entry points, especially their default arguments. Determine why the volume source estimate is hidden unless alpha=0. Done means the volume rendering is visible with the defaults and the reported reproduction no longer shows an opaque inflated brain over it.