Add support for UINT8-based property textures in CustomShader

[ptrgags]

This PR finishes what #10073 started; it implements the basics of property textures (see the EXT_structural_metadata spec) in CustomShader.

Parsing metadata in the shader is rather involved due to type conversions and WebGL 1 limitations. So this PR only implements the following types which are the easiest to represent in texture form:

• SCALAR UINT8 (normalized) -- these become float in the shader
• SCALAR UINT8 -- these become int in the shader since WebGL 1 doesn't support uint
• VEC2/3/4 UINT8 (normalized) -- these become vec2/3/4 in the shader. These are parsed from multiple texture channels.
• VEC2/3/4 UINT8 -- these become ivec2/3/4 in the shader. These are parsed from multiple texture channels.
• fixed-size array of UINT8 (normalized and unnormalized) with 2-4 components -- these become the most appropriate vector type as described in the previous 2 bullets.

Though the spec supports several more types (e.g. a UINT16 spread across two texture channels), those will be handled in future PRs.

Sandcastle for testing

To Do:

• Update CHANGES.md
• Fix typescript build error
• Update CustomShader guide
• Open an issue for remaining steps Handle remaining types for property textures in custom shaders #10248
• Open an issue for noData/default metadata values in CustomShader. Handle noData/default metadata values in CustomShader #10249

[cesium-concierge]

Thanks for the pull request @ptrgags!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ptrgags]

Okay this is ready, @lilleyse could you review?

[lilleyse]

Could you add a new Sandcastle example based on https://demos.cesium.com/owt-uncertainty minus the picking functionality?

[ptrgags]

@lilleyse added the sandcastle, this is ready for review again

[ptrgags]

@lilleyse updated!