Implement VP9 profile 2 - 10 bit BT2020 support with libvpx. This code truncates the 10 bits to 8. We'll later update this to upload half-float or 16 bit short textures.

Pending: Convert BT2020 to DCI-P3 before render.

I'll add the same code to V2 after initial review.

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Created by MOE: https://github.com/google/moe
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