MRG, BUG: Fix vector scaling

[larsoner]

Fixes the bug with glyph colormap not being updated, tested with:

Details

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, 400)
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, background='k')

A few things along the way:

1. Consolidate update_lut so that there is no longer a need for the very repeat-y update_fmin, update_fmid, and update_fmax
2. Refactor some time viewer slider controls to make one call to update_lut instead of potentially multiple calls like update_fmin followed by update_fmid
3. Make update_fscale private (I don't think it was meant to be public)
4. Never apply alpha to the LUT, but instead apply it to the mesh(..., opacity=alpha) instead.

Approximately 3x minus-to-plus ratio on this PR suggesting that it's DRYing out our code. No test added because I'm not sure that there is a great way to test this. In theory if there were existing vector + TimeViewer tests it could be added there, but in a quick check I did not see any. @GuillaumeFavelier feel free to correct me if I'm wrong...

[GuillaumeFavelier]

here were existing vector + TimeViewer tests it could be added there, but in a quick check I did not see any

Its test is test_plot_vector_source_estimates in test_3d.py. The renderer_interactive fixture makes it available for the PyVista backend with time_viewer=True and show_traces='auto'.

[agramfort]

thx @larsoner