In TX size search the rate and distortion values are often computed for
residuals that have already been seen before, with exactly the same
entropy context. Such repeated computations occur when the residue
signal is the same for 2 or more partitions for a given prediction mode.
By saving previous RD search results we can achieve some encoder
speed-up at the cost of higher RAM consumption (~20MB).

Test results with all default experiments:
Speedup      15% (measured on 4 lowres 30-frame sequences)
compression   0%

Change-Id: I8d9b8dd483cf862f0864b50ae80466beac978290