MRG, ENH: Add resizing when time_viewer is present

[larsoner]

On this PR this code snippet correctly sets the render area to the requested size=(800, 600), and yet is resizeable afterward:

MWE

import os.path as op
import mne

data_path = mne.datasets.sample.data_path()
sample_dir = op.join(data_path, 'MEG', 'sample')
subjects_dir = op.join(data_path, 'subjects')
inv = mne.minimum_norm.read_inverse_operator(op.join(
    sample_dir, 'sample_audvis-meg-oct-6-meg-inv.fif'))
evoked = mne.read_evokeds(op.join(sample_dir, 'sample_audvis-ave.fif'))[0]
evoked.apply_baseline((None, 0))
stc = mne.minimum_norm.apply_inverse(
    evoked, inv, method='dSPM', verbose='debug', pick_ori='vector')  # 'vector')
initial_time = 0.1

size = (800, 600)
with mne.viz.use_3d_backend('pyvista'):
    brain = stc.plot(subjects_dir=subjects_dir, initial_time=initial_time,
                     clim='auto', views='lat', hemi='lh',
                     smoothing_steps='nearest', verbose=True,
                     size=size)
sc = brain.screenshot()
assert sc.shape[:2] == size[::-1]

@GuillaumeFavelier can you take over to actually incorporate this properly? For example this also needs to work when time_viewer=False (it does not currently) -- you can probably use similar techniques. But I'll let you decide how to refactor to make it work for both cases.

[larsoner]

@GuillaumeFavelier can you add tests for size? I started to add some but the size was wrong for one run, things don't work with latest pyvista + pyvistaqt, etc. so it was not trivial

[GuillaumeFavelier]

I'll take care of it