Fix crash when two or more windows

[Danile71]

No description provided.

[codecov-io]

Codecov Report

Merging #570 into dev will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              dev    #570      +/-   ##
=========================================
+ Coverage   98.08%   98.1%   +0.01%     
=========================================
  Files          17      17              
  Lines        2092    2112      +20     
=========================================
+ Hits         2052    2072      +20     
  Misses         33      33              
  Partials        7       7

Impacted Files	Coverage Δ
highgui.go	88.88% <100%> (+2.52%)	⬆️

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[Danile71]

I founded bug:
When I start recieving video from 2 cameras and parallel drawing to 2 windows - I have crash.
With sync no bugs

[deadprogram]

Makes sense given the way that OpenCV does all of the UI stuff thru a single thread. I think I tried doing something like this once, not sure why I did not continue?

What OS do you use @Danile71 for testing?

[deadprogram]

This was one of the issues where thread safety was discussed: #191

[Danile71]

I use ubuntu 18

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.10%. Comparing base (d624e2c) to head (e1799ac).
Report is 410 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #570      +/-   ##
==========================================
+ Coverage   98.08%   98.10%   +0.01%     
==========================================
  Files          17       17              
  Lines        2092     2112      +20     
==========================================
+ Hits         2052     2072      +20     
  Misses         33       33              
  Partials        7        7              

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