Add HBD YUV Planar pixel formats

[sandersdan]

This change adds the planar YUV formats described in issue #384, most of which are 10 or 12-bit. (P010/P012/P016/RGBAF16 are not included.)

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Preview | Diff

[padenot]

Generally looks perfect, but I have some questions, that we might be able to address in a follow-up.

[padenot]

I wonder if we should say something about those 6 bits (or 4, depending on the depth) that aren't used.

Are they to be ignored, are they always set to 0? In terms of developer ergonomics, if I read back the values to an ArrayBuffer to perform some computations, should I always mask the unused bits or can I simply read the value with a DataView as uint16_t. In terms of implementation, is that a fair assumption we can make, or do some decoders leave the bits alone, and so this could be a source of fingerprinting?

We should also specify (probably globally, since it applies to all formats) that the sample values are unsigned integer, I tried to look for that but didn't find it for the video side of things.

[sandersdan]

These are fair questions that I don't have answers to. I don't imagine that decoders would do anything other than zero them, but if a frame is created with data in those bits the behavior after that is probably not defined. We may be on the hook to zero them at frame creation, but unfortunately that would prevent using the buffer transfer optimization.

Specifying unsigned integers seems reasonable to me, I'll go ahead and add that in this PR.

[padenot]

I don't know either. I filed #767, let's discuss there once we have implementation experience on various graphics API, hardware and drivers.

This PR looks good to merge from my point of view.

[aboba]

Looks good to me too.