Fixed small bug in cv::Ptr<_Tp> conversion to cv::Ptr<_Tp2>. Now this conversion is made in a more accurate way.

fixing bug #1987(android keeps RGBA in alpha pre-multiplied form, Mat <-> Bitmap should handle this correctly)

fixed possible video distortion problem when cap_avfoundation.mm is used to capture video (ticket #1980)

Added possibility in the class DetectionBasedTracker to work as a tracker only -- without the mainDetector. 
In this case new objects will be added into it by the method addObject(const cv::Rect& location).

which allows to pass as the second parameter of the method set any cv::Ptr<_Tp> object.