fixed iterations>1 case in morphological operations
fixed possible access violation in HSV2RGB
fixed the case learningRate==0 in BackgroundSubtractorMOG2

fixed gpu performance sample

used random images in gpu filter tests

added additional tests for gpu filters
fixed gpu features2D tests

updated gpu tests for CornerHarris and CornerMinEigen
moved direct convolution implementation to gpu::filter2D, gpu::convolve now use only DFT-based algorithm (Bug #1639)

removed void integral(const GpuMat& src, GpuMat& sum, GpuMat& sqsum, Stream& stream) - it fails with NPP_NOT_IMPLEMENTED error
updated docs, accuracy and performance tests

now it supports kernel's size up to 32

switched to gpu::remap in opencv_stitching

GPU: updated upsample, downsample functions, added pyrDown, pyrUp, added support of 16S filtering; put spherical warper on GPU (from opencv_stitching)

changed buffer type in linear filters to float.
added support of 1 channel image to linear filters.
added support of BORDER_REFLECT101, BORDER_REPLICATE and BORDER_CONSTANT border type to gpu linear filters.
minor fix in tests.
update comments in gpu.hpp.

refactored gpu module.

added supports of 4-channels image to gpu::minMax and gpu::cvtColor for RGB <-> YCrCB, RGB <-> YUV and RGB <-> XYZ color conversion.

disabled gpu::sum, gpu::minMax, gpu:Canny until fix crash.

minor fix of matrix_operations.cpp.

added RGB <-> XYZ color conversion.
gpu morphology minor fix.