- add more channels/depth performance test for blend

* Fix the documentation for Mat::diag(int).

Fix issue #8181

* Fix the documentation for Mat::diag(int).

Fix issue #8181.

* Add support for printing out cv::Complex.

* Remove extra spaces.

* cv::Complex is submitted as a new pull request.

Instead of CL_DEVICE_EXTENSIONS in Querying OpenCLVersion, it should be CL_DEVICE_VERSION flag which needs to be passed to the function.

Add support for printing out cv::Complex. (#8208)

* Add support for printing out cv::Complex.

* Conform to the format of std::complex.

* Remove extra spaces.

* Remove extra spaces.

Although both `cl_platform_info` and `cl_device_info` are defined as macro `cl_uint`, it needs to use `cl_platform_info` to get
the platform information.

- don't use undefined flag=0. It should be CONSTANT instead.
- don't allow 'UMat* m=NULL' argument (except LOCAL/CONSTANT flags).
  This case is not handled well to provide NULL __global pointers.
  It is better to use '-D' macro defines instead (at least for performance)

Fix typos in the documentation for AutoBuffer. (#8197)

* Allocate 1000 floats to match the documentation

Fix the documentation of `AutoBuffer`. By default, the following code
```.cpp
cv::AutoBuffer<float> m;
```` 
allocates only 264 floats. But the comment in the demonstration code says it allocates 1000 floats, which is 
not correct.

* fix typo in the comment.

CMake has a long-standing bug/feature (see [here](https://cmake.org/pipermail/cmake/2015-March/060204.html) and reply [here](https://cmake.org/pipermail/cmake/2015-March/060209.html)) which can mangle certain path variables by attempting to make them into relative paths if you try to set them with CACHE PATH.

Say you have your OpenCV download at `/path/on/my/computer/to/opencv/`. What actually happens is that if you try to set this variable by invoking CMAKE with `-DCMAKE_INSTALL_PREFIX=/my/desired/install/path`, what you end up is *not* `/usr/local/` and *not* `my/desired/install/path`, but instead, this monstrosity: `/path/on/my/computer/to/opencv/src/OpenCV-build//my/desired/install/path`.

That is, CMake attempts, for some reason, to turn the path that you passed into a path relative to `${CMAKE_BINARY_DIR}`. See the links I posted above: this is a known (and apparently unfixable) issue with CMake.

In OpenCV's case, among other potential issues, this leads to broken paths in `opencv_tests_config.hpp`, which can break the build or cause bizarre behaviour.

The fix for this issue, as stated in my links above, is to test that the variable hasn't been set yet with an `if(NOT DEFINED ...)` before attempting to set it. This is what I've implemented here. I admit I don't know enough about OpenCV's internals to know whether you *really* need to force the install to be in `/usr/local`, but as it stands right now you get *neither* a clean `/usr/local` path *nor* a customized `/my/desired/install/path`, but a broken mess. This change at least allows the user to customize their install directory.

In the meantime, there's a workaround for this, by explicitly defining the variable as a path with `-DCMAKE_INSTALL_PREFIX:PATH=my/desired/install/path`. But if this change can save anyone else the hours of headaches that I had today, I'll be happy.

 - let it possible to reach Bayer2BGRA conversion

cv2.findContours() no longer modifies source image (#8150)

* cv2.findContours() no longer modifies source image

Since OpenCV 3.2, cv2.findContours() no longer modifies the source image but returns a modified image as the first of three return parameters.  ??? Do I have that correct ???

Also fixed up the code blocks to be valid markdown.

* ```python --> @code{.py}

Enables syntax highlighting in docs.

taken from https://github.com/google/googletest/pull/856