- also some minor changes to the respective sample cpp file

The Mat "src_copy" is never used.

* example using the Canny algorithm with custom image gradient.

* Modified example to integrate new function canny with custom gradient

Program flow is not easily apparent so hopefully ANY comments in the code could help new users.

- use `@snippet` instead of `@code` in docs.
- remove some functions that were not used.

in this case the sample will not be built if OpenGL is not found

1. Explain grayscale input still read as three channel
2. Fix typo
3. Add more details to image match explanation to include the use of masks

change the `while` loop to check if the two windows created by this example still exist, stop the program otherwise

removed binary

update

Moved second `imshow()` inside the if-statement to prevent error when no lines have been found and the function is called with an empty `cdst`.

I noticed that I missed the fact that `cimg` is used in the second `imshow()` call. Changed the scope of the second function call to be within the if-statement. Otherwise in cases where have not been detected the second `imshow()` will attempt to use `cimg` which will be empty leading to an error.

In the C++ equivalent of this example a check is made whether the vector (here in Python we have a list) actually has any lines in it that is whether the Hough lines function has managed to find any in the given image. This check is missing for the Python example and if no lines are found the application breaks.

In the C++ equivalent of this example a check is made whether the vector (here in Python we have a list) actually has any circles in it that is whether the Hough circles function has managed to find any in the given image. This check is missing for the Python example and if no circles are found the application breaks.

fix for #6598

All of these: (performance) Prefer prefix ++/-- operators for non-primitive types.
[modules/calib3d/src/fundam.cpp:1049] -> [modules/calib3d/src/fundam.cpp:1049]: (style) Same expression on both sides of '&&'.

Allow configuration of the square size in stereo_calib.

https://github.com/Itseez/opencv/issues/6456