Generate contours on Cesium3DTileset

[kiril4913]

@John_Mark_Dollosa
you can investigate the silhouette property

Only apply silhouette to a specific 3D tileset

1. A concise explanation of the problem you’re experiencing. I would like to add Cesium.PostProcessStageLibrary.createSilhouetteStage() and only select a specific 3D tileset but i can’t seem to make that work 2. A minimal code example. If you’ve found a bug, this helps us reproduce and repair it. https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=rVTbUtswEP0VTV4SOrHsJAQCGKbXaR8yhWkoTKfpgyxvsIoseSQ5IXT4965s5wbloaV5sfZyds8eaTNnhswFLMCQU6JgQd6BFWVOrypfp80r851WjgkFpt0lv6bqYe9kqqZqjlgnJFhwu+D6M3…