Screen: Add timestamping fallback to DC1394 video capture engine.

libdc1394, as of December 2014, doesn't support timestamping for
captured video from USB-3 IIDC cameras, only from Firewire cameras.
Timestamps from USB-3 are therefore invalid/unuseable.

Add a fallback: If video capture/recording is done on the parallel
background capture thread, instead of synchronously on the main
thread, simple query current "GetSecs" time at time of frame
reception by the capture thread. As the capture thread uses
periodic polling for new frames, this has a minimum timestamp noise
and granularity of the polling interval + random scheduling jitter +
underlying USB-3 jitter/delays. In 'lowlatency' / 'dropframes' mode,
the poll interval is roughly 1 msec, in normal mode it is roughly
4 msecs. Anyway, some kind of rough timestamps might be better than
none at all.

-> Screen for 64-Bit Octave on Linux rebuilt, test pending.