driver/usbvideodriver: reduce latency of GStreamer pipeline

[tretter]

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.

• [x] PR has been tested

Tested by running labgrid-client video on a place with a USB camera as resource, moving in front of the camera, and looking the delay of the movement on the screen.

[Bastian-Krause]

CI errors are unrelated, see #1493.