libvpx

Separate rate-distortion modeling for DC and AC coefficients

This is the first step to rework the rate-distortion modeling used
in rtc coding mode. The overall goal is to make the modeling
customized for the statistics encountered in the rtc coding.

This commit makes encoder to perform rate-distortion modeling for
DC and AC coefficients separately. No speed changes observed.
The coding performance for pedestrian_area_1080p is largely
improved:

speed -5, from 79558 b/f, 37.871 dB -> 79598 b/f, 38.600 dB
speed -6, from 79515 b/f, 37.822 dB -> 79544 b/f, 38.130 dB

Overall performance for rtc set at speed -6 is improved by 0.67%.

Change-Id: I9153444567e5f75ccdcaac043c2365992c005c0c