Circular buffer motion trigger

Add simple circular buffer motion trigger:

"raspivid -trigger 960,540,80,0.5 -o test.h264"

It will look at the macro block containing the coordinate. Calculating the
velocity magnitude and trigger a record event if the magintude is greater
then 80 the last value defines how long to wait after triggering here 0.5
so it will wait 0.5*timeout(=2.5s) before stopping the recording.