Optimized updatebg
Added predicted,measured,updated location to tracks.
Keep snap() array of snapshots, renamed newvis to "vis", update video only when ahead of input stream
Renamed args, compute error from tracks
Update 'updateLoc'
In diagnostic(), plot predicted,measured,updated locs, use 'snap' as argument
Update assignments matrix, measuredLoc, updatedLoc, after adding new track
Set error to NaN if missing predictedLoc or measuredLoc