Screen/dc1394: Add GStreamer post processing support to video capture.

The new 'SetVideoCaptureParameter' setting 'SetGStreamerProcessingPipeline=xxx'
allows to define a gst-launch style GStreamer pipeline which can post-process
captured video before it is returned to the userscript by Screen('GetCapturedImage').

The processing only affects returned video, not recorded video, and it only works
with 'dropframes' = 0 setting and multi-threaded capture, ie., when the GStreamer
pipeline is involved.

Data flow is: appsrc -> caps-filter -> xxx post processing -> appsink -> application.
During video recording, a tee element diverts frames before post-processing to the
video encoding/writing pipeline.

-> Screen fore 64-Bit Octave on Linux rebuilt and tested.
-> VideoMultiCameraCaptureDemo contains a bit of disabled demo code, which shows
   the 'warptv' effect plugin as defined in the variable 'processingString'.

-> This can come handy for post processing tasks like image rotation, gamma correction,
   sharpening/blurring, etc.