libvpx

Refactor block_yrd function for RTC coding mode

This commit separates Hadamard transform/quantization operations
from rate and distortion computation in block_yrd. This allows one
to skip SATD computation when all transform blocks are quantized
to zero. It also uses a new block error function that skips
repeated computation of sum of squared residuals. It reduces the
CPU cycles spent on block error calculation in block_yrd by 40%.

Change-Id: I726acb2454b44af1c3bd95385abecac209959b10