Fixes for VTK layout

[philippjfr]

Fixes #987

[codecov]

Codecov Report

Merging #988 into master will increase coverage by 0.46%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #988      +/-   ##
==========================================
+ Coverage    85.7%   86.17%   +0.46%     
==========================================
  Files          99       99              
  Lines       11239    11418     +179     
==========================================
+ Hits         9632     9839     +207     
+ Misses       1607     1579      -28

Impacted Files	Coverage Δ
panel/pane/base.py	92.07% <0%> (+0.53%)	⬆️
panel/pane/vtk/vtk.py	54.63% <0%> (+2.25%)	⬆️
panel/interact.py	76.07% <0%> (+2.5%)	⬆️
panel/io/save.py	81.35% <0%> (+5.43%)	⬆️
panel/util.py	94.89% <0%> (+7.94%)	⬆️
panel/pane/image.py	86.66% <0%> (+8.09%)	⬆️
panel/pane/media.py	83.19% <0%> (+12.26%)	⬆️

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