Add new demo VideoMultiCameraCaptureDemo.

This is work in progress: A demo for simultaneous videocapture
from multiple connected cameras, with all possible bells and
whistles. Works but is incomplete - this is a backup commit.

Also rebuilt Screen.mex for 64-Bit Octave on Linux for testing.
Screen: Add DC-1394 videocapture device enumeration (wip).

This is a backup commit for work in progress.