When opening a file with GStreamer:

* if the filename looks like a URI, it is opened in non-blocking mode, cvQueryFrame() could skip frames or grab one frame more than once
* if the filename looks like a filename, it is opened in blocking mode. cvQueryFrame() grabs consecutive frames
* otherwise the filename is interpreted as a gstreamer pipeline as used with gst-launch. The last element of the pipeline has to have the property name=to-opencv