The soft coefficient optimization process would monotonically
increase the transform block distortion and decrease the
coefficient rate cost. Such observation provides a lower bound
on the rate-distortion cost for the given transform block. This
commit compares this lower bound against the best available
rate-distortion cost value and skips unnecessary optimization
process. It speeds up the baseline encoding process by 15%.

Change-Id: Ida8098a2820cef60d59ec1e72f0bbb1acbd98165