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

1. Following condition is True on each iteration becuase -1 % 0xFF is 255 not -1

code = cv2.waitKey(100) % 0x100
if code != -1:
   break

this were resetting point position on each cycle not on key press as intended

2. Previous small bug were masking serious bug with matrix operation on matrices of incorrect size.
   As the result on 2nd iteration of internal cycle program has crushed.

   I have fixed it too, matrix operation was taken from examples/cpp/kalman.cpp where it looks like
   randn( processNoise, Scalar(0), Scalar::all(sqrt(KF.processNoiseCov.at<float>(0, 0))));
   which is something totally different from previous code here.

   Example behave as it should now, i.e. point moving by circle trajectory as in C++ example.

responses must be float, else we trigger an assertion in line 150

Added some asserts.

  - previously BB was ignored for first frame, now masks analysis area
  - also allow real-time feedback of processing

Added new test (separating two points)

Also fixed documentation, refactored sample.

Without fixes after 68 line (img1_filename = parser.get<std::string>(0);) OpenCV Error:
./stereo_matching im0.png im1.png --max-disparity=16 --blocksize=17

OpenCV Error: Bad argument (undeclared position 0 requested) in getByIndex, file /home/entodi/opencv/modules/core/src/command_line_parser.cpp, line 169
terminate called after throwing an instance of 'cv::Exception'
  what():  /home/entodi/opencv/modules/core/src/command_line_parser.cpp:169: error: (-5) undeclared position 0 requested in function getByIndex