Fix get scalar array bug for VTK pane

[banesullivan]

There was a bug that would prevent scalars from being shown sometimes. This fixes it.

Here's code to see the difference (running off of vtki master branch - note there are several examples here that show this happening):

import vtki
import numpy as np

vtki.set_plot_theme('doc')
sphere = vtki.Sphere(radius=3.14)

# make cool swirly pattern
vectors = np.vstack((np.sin(sphere.points[:, 0]),
np.cos(sphere.points[:, 1]),
np.cos(sphere.points[:, 2]))).T

# add and scale
sphere.vectors = vectors*0.3

# plot the arrows and the sphere
p = vtki.Plotter()
p.add_mesh(sphere.arrows, scalars='GlyphScale', color=None, lighting=False, stitle='Vector Magnitude')
p.add_mesh(sphere, color='grey', ambient=0.6, opacity=0.5, show_edges=False)
p.show()

[codecov-io]

Codecov Report

Merging #372 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #372   +/-   ##
=======================================
  Coverage   86.63%   86.63%           
=======================================
  Files          92       92           
  Lines        7884     7884           
=======================================
  Hits         6830     6830           
  Misses       1054     1054

Impacted Files	Coverage Δ
panel/pane/vtk.py	30.32% <0%> (ø)	⬆️

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[philippjfr]

Thanks @banesullivan