Support `EXT_textureInfo_constant_lod`

[j9liu]

Feature

EXT_textureInfo_constant_lod is an extension that indicates texture coordinates should be dynamically computed to maintain a consistent texel-to-pixel ratio on screen, regardless of camera distance.

This appears to be an extra computation step that would be enabled by a flag in the shader. However, it may be tricky to implement in our Unreal material since it's computed per-texture. It would be nice if there was a way to avoid adding an if branch for each texture, if possible.

References

• EXT_textureInfo_constant_lod spec
• CesiumJS PR: Implement constant LOD glTF extension cesium#13121