added epsilon value to weights in the MergeMertens in order to avoid zero weights for pixels from uniformly filled areas of image
added test for correct handling of uniforma areas in the MergeMertens
added 3 channels support to cv::bilateralFilter

Conflicts:
	modules/imgproc/test/ocl/test_filters.cpp
added 3-channels support to cv::boxFilter, cv::blur, cv::sqrBoxFilter
added 3-channels support to cv::filter2D, cv::Laplacian
added 3-channels support to cv::medianBlur
refactored filter2D; eliminated restrictions sdepth == ddepth, delta == 0
ocl morphology fix
Merge pull request #2526 from KonstantinMatskevich:ocl_morph_fix
Merge pull request #2123 from white-pony:vkysenko/fix-mertens
Merge pull request #2523 from ilya-lavrenov:tapi_filters