Added support for YUV2RGB[A]_NV21 and YUV2BGR[A]_NV21 conversion
Added OCL code for YUV2BGR_YV12 and YUV2BGR_IYUV color conversions
Added OCL code for YUV2GRAY_420 color conversion
Added OCL code for RGB[A]|BGR[A] -> YUV_[YV12|IYUV] color conversion
Added OCL code for YUV422 -> RGB[A]|BGR[A] color conversion
Used direct float calculations
Optimize OpenCL version of conversScaleAbs function
Added loading 4 pixels in line instead of 2 to RGB[A] -> YUV(420) kernel
Added optimized loading to YUV2RGB_422 kernel
Merge pull request #3354 from vbystricky:oclopt_convertScaleAbs
Merge pull request #3318 from akarsakov:ocl_cvtcolor