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