Allow block skip coding option in RTC mode

When the estimated rate-distortion cost of skip coding mode is
lower than that of sending quantized coefficients, allow the
encoder to drop these coefficients. This improves the compression
performance of speed -6 by 0.268% and makes the encoding speed
slightly faster.

Change-Id: Idff2d7ba59f27ead33dd5a0e9f68746ed3c2ab68