Support color array for *points* type about xviz HOT 5 CLOSED

Especially for the DRACO compression that I am currently experimenting with, (which I am trying to enable on the GLBBuilder level in the new loaders.gl module), it is important that the GLBBuilder doesn't just see one binary array at a time. It needs to be able to identify all the attributes that are part of the point cloud, since the DRACO encoder works on all the attributes at once).

I have a sense that because of this it would be good if a clear pointcloud primitive is defined in XVIZ. Perhaps this is the current point primitive. If not a new one that allows multiple attributes (long vectors of numbers) to be passed in.

It is a total nit, but calling the primitive pointcloud seems more descriptive to me.

@jlisee @twojtasz

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My original thinking for colors was that a single color was a color for all the points, but if you you expanded the list it became a color for each point.

Do we want to flatten the array as well for efficiency?

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I believe we do want to support flattened arrays for both the points and color.
I think points => pointcloud may be worth it as well.

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Support both color, single for all (today) and if >1 it's per point
[rgba]

from xviz.

See #109 to track the implementation of this.

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