- cache value, evaluate once
- better support for MINGW
- anything in 'cv' namespace
- test: dump number of active threads

- FFmpeg 3.4.7

* feature: Add video capture bitrate read-only property for FFMPEG backend

* test: For WIN32 property should be either expected or 0.

Added `IsOneOf` helper function, enabled only for _WIN32.

- Android Manager version is not increased (stuck on 3.49)

dnn(darknet-importer): add grouped convolutions, sigmoid, swish, scale_channels

* update darknet importer to support enetb0-yolo

* remove dropout (pr16438) and fix formatting

* add test for scale_channels

* disable batch testing for scale channels

* do not set LayerParams::name

* merge all activations into setActivation

* Add Tengine support .

* Modify printf to CV_LOG_WARNING

* a few minor fixes in the code

* Renew Tengine version

* Add header file for CV_LOG_WARNING

* Add #ifdef HAVE_TENGINE in tengine_graph_convolution.cpp

* remove trailing whitespace

* Remove trailing whitespace

* Modify for compile problem

* Modify some code style error

* remove whitespace

* Move some code style problem

* test

* add ios limit and build problem

* Modified as alalek suggested

* Add cmake 2.8 support

* modify cmake 3.5.1 problem

* test and set BUILD_ANDROID_PROJECTS OFF

* remove some compile error

* remove some extra code in tengine

* close test.

* Test again

* disable android.

* delete ndk version judgement

* Remove setenv() call . and add License information

* Set tengine default OFF. Close test .
Co-authored-by: Vadim Pisarevsky <vadim.pisarevsky@gmail.com>

Lets the user choose the maximum number of iterations the robust
estimator runs for, similary to findHomography. This can significantly
improve performance (at a computational cost).

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.