libvpx

Account for chroma component costs in RTC mode decision

This commit allows the encoder to account for additional chroma
plane costs in the mode decision process, if the current block
potentially contains significant color change. It improves the
visual quality at very low bit-rates.

The compression performance of dark720p is improved by 12.39% in
speed 6. For jimred at 150 kbps, the PSNR of V component (red)
increased by 0.2 dB, at the expense of about 5% increase in
encoding time. Note that for sequences where the chroma components
are fairly consistent, the encoding time increase is negligible.

On average the rtc set compression performance is improved by
1.172% in PSNR and 1.920% in SSIM.

Change-Id: Ia55b24ef23a25304f7ec9958fbf07fd6e658505c