Experimental change to two pass prediction decay calculation.
Remove dependency on amount and speed of motion as this may not behave well across different image sizes. Tweak impact of % inter. Add in experimental adjustment based on relative quality of an older second reference frame. Cap range of decay values allowed. Some small + effect on derf but -ve on yt & hd at this stage. Change-Id: I390d6f6ebe67a2eb0b834980d0d4650124980d3e