3D Tiles - sign, floor, ceil, round, etc for styling language

[lilleyse]

For CesiumGS/3d-tiles#2 and #3241

@leesafini After #4556 is done let's add these functions to the styling language in a single PR:

• sign(x)
• floor(x)
• ceil(x)
• round(x)
• exp(x)
• log(x)
• exp2(x)
• log2(x)
• fract(x) - glsl has a built-in function for this, but JavaScript Math does not - you'll need to come up with a good command for doing this in JS

Also add these functions to the 3D Tiles spec in a single PR.

In the 3D Tiles Point Cloud Styling Sandcastle feel free to use multiple functions in the same style. For example floor, ceil, and round could all go together, exp, log, exp2, log2 could also go together.

[Dylan-Brown]

@lilleyse Is this issue finished? They all seem to be 'ticked' in #4865; another related issue or two also seems finished

[lilleyse]

Yup it is, thanks for checking.