Add procedurally-generated billboard clouds

[j9liu]

This PR implements the cloud enhancement as outlined in #9691, including the following features.

• New CumulusCloud class and CloudCollection API, with a similar interface to BillboardCollection
• Procedural cloud textures based on a combination of techniques in Gardner's Visual Simulation of Clouds and the Real-Time Volumetric Cloudscapes of Horizon: Zero Dawn.
• Parameters in both CumulusCloud and CloudCollection that the user can tweak to vary the appearance of their clouds
• "Cloud Parameters" Sandcastle (localhost) that demonstrates how each parameter affects the cloud.

This is still a work in progress. Remaining tasks:

• Add images to CumulusCloud and CloudCollection documentation.
• Create a Sandcastle showcasing multiple clouds in a non-empty scene.

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

@j9liu great job on this! The approach you went with strikes the right balance of ease of use and flexibility for advanced users. Excellent doc and unit tests like usual. Mostly minor feedback from me on the code side.

While it would be great to have fully procedural, volumetric clouds in the future this is a great start. I'm looking forward to your roadmap issue.

[lilleyse]

Some more meta feedback - now that I've reviewed the code I think CloudCollection could possibly contain a BillboardCollection under the hood, but there would have to be some changes to get it to work:

• BillboardCollection deals with both the position/sizing/orientation of billboards and texture atlasing. These two aspects would need to be decoupled since CloudCollection only needs the first as it does its own procedural texturing.
• BillboardCollection would need to allow for custom attributes - like cloud brightness and the various noise parameters
• BillboardCollection would need to support a shader hook in point in the frag shader, so that CloudCollection can insert its own procedural shading

Given the extent of these changes the current approach still makes sense, but if we want to consolidate in the future I think we can.

[j9liu]

@lilleyse - just made some changes, let me know what you think!

[lilleyse]

I pushed some small tweaks in 41db4a8 and updated CHANGES.md.

Thanks again @j9liu