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