videoio: refactor GStreamer backend

- exception safe smart pointers, fixed many memory leaks
- fixed errors handling
- fixed strings handling
- fixed file existence check
- fixed toFraction() implementation
- add 'drop=true' for v4l2 input
- avoid overriding custom pipelines sink's 'max-buffers' value
- updated valgrind supression rules
- added support for GStreamer native leaks tracking (via gst_deinit() call)
- repaired capturing through GStreamer0.10 (fixed CV_8UC1)

modules/videoio/src/precomp.hpp

@@ -154,8 +154,11 @@ CvVideoWriter* cvCreateVideoWriter_AVFoundation( const char* filename, int fourc
 
 CvCapture * cvCreateCameraCapture_Unicap  (const int     index);
 CvCapture * cvCreateCameraCapture_PvAPI  (const int     index);
+
+namespace cv {
 CvVideoWriter* cvCreateVideoWriter_GStreamer( const char* filename, int fourcc,
                                             double fps, CvSize frameSize, int is_color );
+}
 
 
 namespace cv