fixed slow down in getRectSubPix (original patch by Markus JR Mueller); removed some unused variables and functions

changed the way the mean value for each pixel is assigned in the output image.

modified BackgroundSubtractorMOG2::getBackgroundImage so that it can now work with gray-level images.

1. decrease branch number in CL code by replacing them into weights
2. decrease local mem pressure in reduce operation by using private variables
3. decrease image sampler pressure by caching data into local memory
4. remove unnecessary sync point on the HOST side.

Fixed an issue with weight assignment causing the resulting GMM weights to be unsorted in BackgroundSubtractorMOG2

fix for cv::LUT and cv::transpose