XVision now supports synchronous framegrabber operation as follows:

    There is a new call "forward" at the Video level which is designed
    to be the function that "advances" to the next frame.  This
    is probably the same code that used to go into "pre-grab".

    Each tracker now has "update" and "state_update".  Update is
    the tracker-specific updating code.  State_update does the
    additional booking to check on the synchronization with the
    video source.  All the XVision code as been adjusted to use
    state_update where appropraite.

    Each video device now has a "set_synchronous(bool)" call which
    turns framegrabber syncronization on and off.

This has been implemented for the MPEG device.  For example, calling
SSDtest with the -M flag creates a synchronous MPEG device; likewise
for the -mpeg flag on cornertest.

SSDtest has also been modified to have a set_oiter call with sets the
"outer" iterations on teh SSD update.  Hence, using oiter > 1 and
set_synchronous(1) is advised.

This is all implemented by keeping track of frame counts in the video
device and the trackers.  Probably a bit brittle --- buyer beware!!

GDH 9/4/98
