This commit re-works the rate-distortion optimization scheme for
transform coding. It improves the overall compression performance.
For derf set, the ext-tx experiment provides 2.27% coding gains,
and the new scheme that integrates multiple transform type selection
and recursive transform block partitioning provides a total of 3.24%
coding gains.

Change-Id: Ia1887c4c44b73dfb915d091d96660a99f09d5cc3