Screen/dc1394: Allow to set 'StopAtFramecount' before capture is started.

We use an intermediate latch variable to allow either changing the limit
while capture is running, as before, or to set the limit at 'StartCapture'.

-> More flexibility, less race conditions.
-> Screen for 64-Bit Octave on Linux rebuilt and tested.
Screen/dc1394: Allow use of background thread for regular video capture.

recordingflags & 16 can now request use of the background processing
thread even if video recording isn't active. This allows to offload
computations like debayering and color conversion in pure video capture
configurations.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> VideoMultiCameraCaptureDemo uses threaded capture atm to test this.
Screen/dc1394: Add video frame timestamping support during recording.

If the 'movieoptions' flag 'UseVFR' is used, then "Variable FrameRate"
recording is enabled: Each video buffer is timestamped with its video
capture timestamp from libdc1394 capture before it gets pushed into
the GStreamer video recording pipeline. The pipeline omits the videorate
converter element which would normally normalize the timestamps to
correspond exactly to the nominal framerate of the recording.

-> The recorded video contains timestamps which are still zero-based,
   as they count time since start of movie, but the individual frame
   timestamps are no longer synthetic, but from the real live video
   capture process, so they should capture potential variability or
   camera sync issues.

-> Regular movie writing via Screen('CreateMovie') always uses
   videorate converter + synthetic timestamps.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> VideoMultiCameraCaptureDemo uses 'UseVFR' by default during recording
   to test this.