Add ocl version BackgroundSubtractorKNN (#10553)
* Add ocl version bgfg_knn
* Add ocl KNN perf test
* ocl KNN: Avoid unnecessary initializing when non-UMat parameters are used
* video: turn off OpenCL for color KNN on Intel devices
due performance degradation
* video: turn off KNN OpenCL on Apple devices with Intel iGPU
due process freeze during clBuildProgram() call
The method does cvCheckPixelBackgroundNP (which reads bgmodel)
before it is ever updated (in cvUpdatePixelBackgroundNP). This
initialization is thus needed to avoid reads of unitialized values.