video: add a default frame latency of 1 frame

This is configureable in adancedsettings under video/latency/frames.
The default value should correspond to a A/V delay of -16ms at 60 hz
and -41ms at 24hz

Most video output will be counting it's delay in times of frames, not
in absolute time. Any advanced frame processing on normal TV's will
at least incure a 1 frame delay.