Sets the reference
frame interval where n refers to a reference frame (I-frame) being
inserted every nth frame. A value of 0 results in no I-frames, 1
causes every frame to be an I-frame, 2 causes every other frame to be an
I-frame, and so on. Although there is no upper bound on the reference frame
interval, a reference frame interval that does not exceed two seconds produces
the best results. The default reference frame interval is every 15th frame
(once a second at the default frame rate of 15 frames per second).