Calculate rd cost per transformed block

Compute the rate-distortion cost per transformed block, and cumulate
the cost through all blocks inside a partition. This allows encoder
to detect if the cumulative rd cost is already above the best rd cost,
thereby enabling early termination in the rate-distortion optimization
search.

Change-Id: I0a856367a9a7b6dd0b466e7b767f54d5018d09ac