Adds a speed feature to conduct a brute force search among a set of
available interpolation filters for the best one in an RD sense.

There is a gain of 0.4% on derf, 1.0% on Std-HD.

Patch 2: A macro added to determine if the encoder state is reset
for each new filter tried.

Patch 3: rebase, also fixes a bug (decodframe.c) introduced by a
couple of missing function pointer assignements.

Patch 4: rebase.

Change-Id: Ic9ccca9d8c35c6af557449ae867391a2f996cc29