fixed positioning in ffmpeg wrapper (thanks to mike_at_nrec for the patch). Added frame positioning feature to laplace demo to test the feature

fixed positioning in ffmpeg wrapper (thanks to mike_at_nrec for the patch). Added frame positioning feature to laplace demo to test the feature

added square_size cmd line param for calibrate.py
added _coverage.py script to compute cv2 api coverage by samples (just for interest, may be removed later)

fixed warnings in gbt; added insertChannel() and extractChannel(); made the code "rand{u|n}(arr, <number>, <number>)" work properly.