Adds support for switchable interpolation filters.
Allows for swtiching/setting interpolation filters at the MB level. A frame level flag indicates whether to use a specifc filter for the entire frame or to signal the interpolation filter for each MB. When switchable filters are used, the encoder chooses between 8-tap and 8-tap sharp filters. The code currently has options to explore other variations as well, which will be cleaned up subsequently. One issue with the framework is that encoding is slow. I tried to do some tricks to speed things up but it is still slow. Decoding speed should not be affected since the number of filter taps remain unchanged. With the current version, we are up 0.5% on derf on average but some videos city/mobile improve by close to 4 and 2% respectively. If we did a full-search by turning the SEARCH_BEST_FILTER flag on, the results are somewhat better. The framework can be combined with filtered prediction, and I seek feedback regarding that. Rebased. Change-Id: I8f632cb2c111e76284140a2bd480945d6d42b77a