added video stream positioning in QTKit-based VideoCapture backend (patch #1420)

modules/highgui/src/cap_qtkit.mm

@@ -864,6 +864,17 @@ bool CvCaptureFile::setProperty(int property_id, double value) {
 		case CV_CAP_PROP_FPS:
 			//etval = currentFPS;  
 			break; 
+		case CV_CAP_PROP_FRAME_COUNT:
+			{
+			NSArray *videoTracks = [mCaptureSession tracksOfMediaType:QTMediaTypeVideo];
+			if ([videoTracks count] > 0) {
+				QTMedia *media = [[videoTracks objectAtIndex:0] media];
+				retval = [[media attributeForKey:QTMediaSampleCountAttribute] longValue];
+			} else {
+				retval = 0;
+			}
+			}
+			break; 
 		case CV_CAP_PROP_FOURCC:
 		default:
 			retval = false;