Clean up Custom Shader inputs for voxel rendering

[jjhembd]

Description

This PR cleans up the FragmentInput struct for voxel custom shaders, to be more consistent with other uses of CustomShader and the Custom Shader Guide.

Key changes include:

• Moved metadata statistics to a higher-level fragmentInput.metadataStatistics sub-struct, instead of appending them to each metadata property in the fragmentInput.metadata struct. This brings voxel metadata up to date with the changes to Model metadata in Add shader struct with metadata statistics #10683.
• Added the fragmentInput.attributes sub-struct. It currently only contains positionEC and normalEC.
• Removed several unused properties from the fragmentInput.voxel sub-struct.
• Added the property fragmentInput.voxel.distanceToDepthBuffer as a first step toward resolving Access both voxels and mesh surfaces from a custom shader #12599
• Cleaned up obsolete uniforms and transforms in VoxelPrimitive

Issue number and link

Resolves #11832.

Testing plan

• Load the Voxel Picking Sandcastle and verify that voxel metadata (cell color) and lighting calculations are unchanged

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[jjhembd]

As part of addressing #12599, FragmentInput.voxel now includes a distanceToDepthBuffer property, indicating the distance from the current raymarching position to the currently loaded depth buffer. The depth value will represent the distance to the nearest rendered pixel from the terrain and any Cesium3DTilesets in the Scene.

Here is a basic local Sandcastle rendering the value of the new depth property.

[ggetz]

Overall this is looking great @jjhembd. I left one comment to make sure we are providing any existing users with the info they need to update.

I also pushed a small update which linked to the custom shader guide from the inline ref doc, just to make sure the dots are connected.

[ggetz]

Could we document a list (or if too long, an example or two) of what attributes changed so that users have a place to start when updating?

[jjhembd]

@ggetz thanks for the feedback! I added info and an example to the CHANGES.md. This should be ready for another look.

[ggetz]

Looks great! Thanks @jjhembd!