Additions to Point Cloud spec

[j9liu]

The Point Cloud guide doesn't mention anything about its colors being stored in sRGB color space. This leads to unexpected rendering differences when migrating from pnts to glTF, because a glTF stores colors in linear RGB.

To minimize future confusion, I added some lines about point clouds storing colors in sRGB, and how they must be converted to linear RGB for glTFs.

I also added a line requiring POINTS_LENGTH to be greater than zero, as suggested in #711 (comment).

@javagl could you review? Feel free to rewrite my lines if they are not clear enough.

[javagl]

Looks fine for me. I'll read a little bit more about the context (and what e.g. CesiumJS is doing with the vertex colors), and probably merge soon.

Interstingly, a quick search for srgb does not bring up any mention in this repo. And websearching for 3d+tiles+"srgb" brings up a fairly recent presentation with the lines

Point clouds

Vertex color sRGB (?)

so I guess that "(?)" may become a "(!)" soon...)

[javagl]

I looked a little bit at the handling of the color spaces in CesiumJS, and it seems like there is a dedicated step for converting the sRGB colors from point clouds into the linear color space. One could zoom pretty deeply into that, and ask questions about the 'Styling' section of the specification, but that may be unrelated to this PR.