Webrender: HDR video support

[nical]

With AV1 my understanding is that HDR video is going to become a thing we have to support. There are two levels of support:

• HDR YUV video frames upload and rendering, with a shader that converts outputs non-HDR content,
• HDR compatible pipeline that can leverage HDR displays.

I was told that supporting HDR in the shader is more involved than our current non-HDR "just multiply by this matrix" color conversion code and that VLC uses a somehting that generates shaders on the fly for HDR video.

[gw3583]

https://github.com/servo/webrender/pull/3113 handles the first part of this task.