Fix orientation slider when hemi=both

[GuillaumeFavelier]

This PR fixes the overlap of the sliders when hemi=both.

It's an item of #7162

[agramfort]

can you share a test script to check/replicate?

[GuillaumeFavelier]

Running the following:

import os
import mne
from mne.datasets import sample
data_path = sample.data_path()
sample_dir = os.path.join(data_path, 'MEG', 'sample')
subjects_dir = os.path.join(data_path, 'subjects')
fname_stc = os.path.join(sample_dir, 'sample_audvis-meg')
stc = mne.read_source_estimate(fname_stc, subject='sample')
with mne.viz.use_3d_backend('pyvista'):
    brain = stc.plot(subjects_dir=subjects_dir,
                     clim=dict(kind='value', pos_lims=[3, 6, 9]),
                     time_viewer=True, hemi='both')

This is what I get:

master	PR