3D Tiles Styles and Custom Shaders should not crash on undefined properties

Feature

A given 3D tileset can contain multiple models. Each model may have different metadata - and reference different metadata schema. Yet a tileset, loaded into CesiumJS, can only have a single Cesium3DTileStyle, and a single CustomShader. This makes it difficult to style or apply custom shaders using properties that exist on some models in the tileset but not others. In both cases (styles and shaders), use of a property that's not defined on every model will crash CesiumJS.

My proposal is that undefined properties should not crash styles and shaders. Instead, they should simply be skipped over. I'm not sure yet how that would work for styles (maybe it's easy), but I've looked into the CustomShaderPipelineStage, and I think we should expose the concept of preprocessor macros to the end user. Something like:

const myCustomShader = new Cesium.CustomShader({
  vertexShaderText: `
    void vertexMain(VertexInput vsInput, inout czm_modelVertexOutput vsOutput)
    {
#if HAS_PROPERTY(myDisplacementProperty)
        vsOutput.positionMC += vsInput.attributes.normalMC * vsInput.metadata.myDisplacementProperty;
#endif
    }
  `,
});

If implemented properly, I think this approach could open the door for other user-accessible custom shader preprocessors in the future, should we need them.

Start at CustomShaderPipelineStage and trace the corresponding 3D Tiles style path; these are the only entry points named, and no files or tests are specified. Establish how properties missing from some models should behave, then verify that both styles and custom shaders skip undefined properties without crashing.