driver/usbvideodriver: reduce latency of GStreamer pipeline

The playbin3 uses a urisourcebin to open and read the uri. The
urisourcebin detects the fd as a stream uri and internally crates a
multiqueue with a default buffer duration of 5 s and a high watermark of
0.60 to buffer the stream. This adds 3 s latency to the video stream.

If the source signals that is it a live source, the urisourcebin would
avoid buffering of the stream. Unfortunately, the fdsrc doesn't signal
that the data is live, and doesn't have a property to change this.

Use buffer-duration to set the size of the queue for buffering to 0 to
avoid this additional latency.

The queue could be completely removed by setting "buffering" in the
flags property of playbin3 to 0, which would be equal to the behavior of
a live source, but the changed flags are more difficult to understand
than the buffer duration.

labgrid/driver/usbvideodriver.py

@@ -131,7 +131,7 @@ def stream(self, caps_hint=None, controls=None):
         tx_cmd = self.video.command_prefix + ["gst-launch-1.0", "-q"]
         tx_cmd += pipeline.split()
         rx_cmd = ["gst-launch-1.0"]
-        rx_cmd += "playbin3 uri=fd://0".split()
+        rx_cmd += "playbin3 buffer-duration=0 uri=fd://0".split()
 
         tx = subprocess.Popen(
             tx_cmd,