This commit refactors the rate-distortion optimization scheme for
transform block coding. When both ext-tx and var-tx experiments
are turned on, the encoding time for bus_cif at 1000 kbps goes down
from 706377 ms to 666503 ms (5.6% speed-up). The coding statics
remain unchanged.

Change-Id: I20835db573725580aad79c16220f799ce01f2093