Limit arf interval for low fpf clips.
This patch limits the maximum arf interval length to approximately half a second. In some low fps animations in particular the existing code was selecting an overly long interval which was hurting visual quality. For a sample problem test clip (360P animation , 15fps, ~200Kbit/s) this change also improved metrics by >0.5 db. There may be some clips where this hurts metrics a little, but the worst case impact visually is likely to be less than having an interval that is much too long. On more normal material at 24 fps or higher, the impact is likely to be nil/minimal. Change-Id: Id8b57413931a670c861213ea91d7cc596375a297