This commit addes a new strategy to reduce the search for optimal
interpolation filter type. The encoder counts and store how many each
filter type is selected and used for each of the reference frames.
A filter type that is rarely used for all three reference frames is
masked out to avoid computation.

The impact on compression is neglectible:
-0.02% on derf
+0.02% on stdhd

Encoding time is seen to reduce by 2~3%.

Change-Id: Ibafa92291b51185de40da513716222db4b230383