CvVideoWriter_AVFoundation_Mac had a serious buffer release bug.

Also made writeFrame() block until isReadyForMoreMediaData rather than
return an error.

Support setting CAP_PROP_MODE to capture grayscale or YUV frames much
faster from CV_CAP_AVFOUNDATION_MAC.

resolve undeclared identifier close error

Fix for incorrect calcOpticalFlowPyrLK result evaluation with ARM NEON

UIImages with alpha were ending up with garbage pixels in background (random memory values).  Need to initialize matrix pixels before drawing UIImage with alpha on it.

Note: didn’t fix Grayscale image with alpha stripping alpha in UIImage -> Mat conversion.