Improve ocl cvt_color performance for the following conversions: RGB<->BGR, RGB->Gray, RGB<->XYZ, RGB<->YCrCb, RGB<->YUV, and mRGBA<->RGBA.
The improvement was done basically by processing more than 1 pixel by each work-item and using vector's operations.
new performance tests were added

Added "sep_filter_singlepass" kernel that performs separable filtering in one kernel call
Added appropriate host part - sepFilter2D_SinglePass function and SingleStepSeparableFilterEngine_GPU class
Changed function declarations to enable their usage

The method ocl::oclMat::ptr() is being removed because it returns a pointer to a row of oclMat::data. The data attribute is a cl_mem structure and cannot be iterated outside an OpenCL kernel.

The oclMat::ptr() method was mistakenly used in ocl::BruteForceMatcher to pass a pointer to a oclMat object. The ptr() method returns a uchar pointer to the cl_mem data structure and this method will be removed.

There are some mixed implicitly/explicitly type conversion between
scalar and vector. Although the spec allows those conversion, I prefer
to make them consistent and use explicitly all the cases.
Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>

fixed many bugs related to Mat::getUMat(), asynchronous kernel execution etc. Also, played a bit with ocl::cvtColor vs cv::cvtColor performance