Usage of imread(): magic number 0, unchecked result
* docs: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()
* samples, apps: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()
* tests: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()
* doc/py_tutorials: check imread() result
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.