Chroma weighted prediction

Chroma weighted prediction
Like luma weighted prediction, dramatically improves compression in fades.
Up to 4-8db chroma PSNR gain in extreme cases (short, perfect fade-outs).
On actual videos, helps up to ~1% overall.
One example video with a decent number of fades (ef OP): 0.8% bitrate reduction overall, 7% bitrate reduction just counting chroma.
Fixes a lot of artifacts in fades at lower bitrates.

Original patch by Dylan Yudaken <dyudaken@gmail.com>.