Color shift on cv::cudacodec::VideoReader when using multiple threads reading videos from different color spaces in parallel

[laurentchicoine]

Following latest updates to fix YUV Conversion #3828, I started to notice an issue with some video colors that is flickering. The issue only happens when launching multiple video readers from multiple threads on a single process (using Python) and when these videos have a different color space for example some videos with yuvj420p (BT470) and others in yuv420p.

In my use case, as a video is being read, new VideoReader are being created on seperate threads.

When investigating the issue I found that this is caused because the code is using a global constant matYuv2Color that is being updated in SetMatYuv2Rgb with call to cudaMemcpyToSymbol. https://github.com/opencv/opencv_contrib/blame/eebd8977e158d81a4aaf41653a038b82cf6507d4/modules/cudacodec/src/cuda/ColorSpace.cu#L11

So therefore instanciating a new video reader seems to alter that common constant in some way causing a shift of color since the values in the matrix are different for the other video. I'm not yet 100% sure this is the issue since I did not yet work on fixing it.

I'm unsure how to best resolve this optimally for a proper PR.

@cudawarped since you worked on the original fix.