rc: use a sliding window to calculate moving avg SatdCost for ABR Reset logic

contains improvements for detection of scene changes within Rate Control to stabilize qp and prevent
extreme spikes in bitrate. Removes the blockiness or distortions in the frames that a streak of low-detailed
frames and prevents vbv from overreacting at the points of scene cuts.
doc: make it clear fast cbf only affects rdlevel 5 and 6