Screen/dc1394: Optimize away GStreamer pipeline if no-recording and recording flags = 4.

If usercode doesn't want to use Screen('GetCapturedImage') ie., recordingflags & 4,
and no video recording is active, then disable GStreamer pipeline feedback path.

Shaves off another few fractions of a msec and some threads.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.