More robust initialization of the vtk window

[adam-urbanczyk]

Maybe fixes this problem
#1366 (comment)

[codecov]

Codecov Report

Merging #1394 (24e5469) into master (e8d7447) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 24e5469 differs from pull request most recent head ac21e5a. Consider uploading reports for the commit ac21e5a to get more accurate results

@@           Coverage Diff           @@
##           master    #1394   +/-   ##
=======================================
  Coverage   94.13%   94.13%           
=======================================
  Files          27       27           
  Lines        5678     5678           
  Branches      962      962           
=======================================
  Hits         5345     5345           
  Misses        199      199           
  Partials      134      134           

Files Changed	Coverage Δ
cadquery/vis.py	96.55% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[jmwright]

I think this looks good @adam-urbanczyk but it did not seem to fix the problem for users in the linked issue. However, I don't think this change will hurt anything, and could eliminate a possible race condition on initialization.

[adam-urbanczyk]

I'll merge then.