Screen/dc1394: Add support for execution of 2D marker tracker plugin.

Add two new subfunctions to Screen('SetVideoCaptureParameter') for
the libdc1394 firewire video capture engine on Linux and OSX:

'LoadMarkerTrackingPlugin=' allows to specify a marker tracker plugin,
which gets loaded and linked into the Screen() function, and sub-
sequently executed by the video frame processing thread during
Firewire video capture operations. The plugin must be implemented
by a shared library. We leave the supported API/ABI unspecified
for now, as this is a highly experimental feature which will change
without notice or any backwards compatibility. It is meant for
prototyping some new interesting future applications.

'SendCommandToMarkerTrackingPlugin=' allows to send ASCII strings
with commands to the loaded tracker plugin, to configure its
operation.

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