ToT libc++ (LLVM) no longer includes <sstream>
as part of <complex> which breaks building opencv.
Include <sstream> header explcitly to fix this.

* core(logger): add CV_LOG_ONCE_xxx() CV_LOG_IF_xxx() macros

* core(logger): keep tests disabled

* imgproc(integral): avoid OOB access

* imgproc(test): fix integral perf check

- FP32 computation is not accurate

* imgproc(integral): tune loop limits

clGetSupportedImageFormats returns CL_INVALID_VALUE if called with
num_entries 0 and a non-NULL image_formats pointer so let's not do that.

Use argument value for 'mat' in call to format for vector_mat and vector_mat_template

* Supported Div op for constants

* Added Mul test

The hard-coded string value "Mat" was used in the two format strings for vector_mat and vector_mat_template, preventing UMat arguments to functions that have these types from working correctly. as noted in #12231.

- CMake option: OPENCV_DNN_IE_NN_BUILDER_2019

core: fix MatExpr::size() for gemm()

* core(test): MatExpr::size() test for gemm()

* core: fix MatExpr::size() for gemm()

* Ignore clang warnings for deprecated enum+enum operations in mat.inl.hpp

* build: added customization macros, cmake flags for OpenCV build

V4L2: Add V4L2_PIX_FMT_Y12 (12 bit grey) support

* Vectorize calculating integral for line for single and multiple channels

* Single vector processing for 4-channels - 25-30% faster

* Single vector processing for 4-channels - 25-30% faster

* Fixed AVX512 code for 4 channels

* Disable 3 channel 8UC1 to 32S for SSE2 and SSE3 (slower).  Use new version of 8UC1 to 64F for AVX512.

- standard huffman tables are handled by modern libjpeg-turbo

in sample codes instead of cpp files' name.

removed all the changes occured while trying to resolve conflicts

* Custom layers with nGraph

* nGraph: multiple outputs from nodes