Handle multiple sets of feature Ids in ModelExperimental

[ptrgags]

Overview

Right now, ModelExperimental only uses feature IDs for the following:

• Using one selected feature ID for picking/styling. This doesn't always work, Model EXT_mesh_features: handle feature IDs without a property table #9884 needs to be addressed.
• feature ID attributes (only the explicit FEATURE_ID_n ones) show up in custom shaders as fsInput.attributes.featureId_n

However, we also want to support the following:

• consistent method for referencing feature IDs. Each primitive has a list of feature ID sets (some from attributes, some from an implicit range, some from a texture). in the API "feature ID n" or "instance feature ID n" should refer to the n-th entry in these lists of feature IDs
• multiple feature IDs in custom shaders. This should be seamless, the user shouldn't need to worry about texture reads
• Longer term, having the feature IDs accessible will be an important prerequisite for GPU Metadata Property Table Packing for 3D Tiles Next #9572 and Model: GPU Styling #9944

Here are some initial notes of things that'll need to be updated. There's likely more details to consider as I implement this.

#9884 should be addressed first, then this issue.

CustomShader

• Add parsing logic for [fv]sInput.featureIds.(featureId|instanceId)_(\d+)
• featureId_n corresponds to a position within a primitive's featureIds array
• instanceId_n corresponds to a position within the instance's featureIds array

FeatureIdPipelineStage

• Maybe split into a ActiveFeaturePipelineStage and FeatureIdPipelineStage
• Move this stage earlier in the pipeline
• Change this stage to process all feature IDs
• Generate implicit attributes and use a prefix in the shader
• Add a FeatureIds dynamic struct
• Attributes: featureIds.(feature|instance)Id_n = a_featureId_m
• Implicit attributes: featureIds.(feature|instance)Id_n = a_implicitFeatureId_m
• Textures (FS only): featureIds.(feature|instance)Id_n = floor(texture2D(FEATURE_ID_TEXTURE, FEATURE_ID_TEXCOORD).FEATURE_ID_CHANNEL * 255.0 + 0.5);

CustomShaderPipelineStage

• Add FeatureIds to vsInput and fsInput
• Warn if a texture feature ID is used in the vertex shader

ModelExperimental

• Replace featureIdAttributeIndex, featureIdTextureIndex with featureIdIndex and instanceFeatureIdIndex

ModelExperimentalFS

• Add a note that Feature feature; is global-scoped because of its use in picking (maybe also wrap in a #ifdef clause?)