Screen: Fix Format-7 framerate reporting for dc1394 video capture.

"Fix" it by not querying the cam in Format-7 mode, but just using
the calculated nominal value. All tested fw cams report zero = unsupported,
so nominal fps is used as fallback, except one cam which reports completely
bogus values.

This fix allows to get a more proper recording framerate for video
recording to movie files if Format-7 is selected.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
Screen: Add option to prefer Format-7 capture in dc1394 video capture engine.

A new Screen('SetVideoCaptureParameter', grabbers(i), 'PreferFormat7Modes', 1);
setting allows to tell the engine it should prefer Format-7 capture modes over
Non-Format-7 ones, even those would do for given resolution, framerate and ROI.

Why? Because sometimes a Format-7 mode allows to get the same framerate/resolution
and color precision at a slightly lower firewire bus bandwidth consumption.
This setting is therefore useful if one wants to squeeze out maximum performance
or the maximum number of cameras on a given bus.

Default is old behaviour of auto-selecting the mode and preferring non-Format-7
modes if they will do, as Format-7 modes are only supported by higher end gear.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> VideoMultiCameraCaptureDemo.m tests this by capturing video from a
   dual-fw-camera setup at 640x480 pixels resolution at 60 fps, which
   would not be quite possible in non-Format-7 mode due to slightly higher
   bus utilization.
Screen: Add "stop of capture" framecount barrier to dc1394 capture.

We need to make sure that when synchronized stop of capture on
multi-cam synced setups is requested, that all recording threads
dequeue and push the same number of frames from the engine before
terminating, so we don't end up with different final framecounts
in the video files between different cameras.

This adds an implementation which uses the framecount at end of capture
of the master as reference value for all threads to stop.

The target stop count can also get manually queried and set, as
well as the current and possible capture counts.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> Demo updated for testing.
-> Lightly tested with bus sync and seems to work there. More tests tbd.