Tuned restrictions for Canny, Warp, FAST, Accumulate and Convolution OpenVX HAL calls on small images

 - check boundary strictly
 - initialize the variable before using it

- Fix StereoBM ROI (Region of interest) selection for stereo
images used by block matching algorithm

The docstring for one of the Python sample programs includes a link to the research paper describing the main algorithm. That link is no longer valid (results in a 404 error) so this update replaces it with another link from the same institution which is currently valid.