Add console warnings when a tileset contains unsupported extensions

[ptrgags]

If a tileset specifies an extension in extensionsRequired, that is not supported, we should log at least a warning in the console.

• We'll need to put together a list of supported extensions:
  • 3DTILES_metadata
  • 3DTILES_implicit_tiling
  • 3DTILES_content_gltf
  • 3DTILES_multiple_contents
  • 3DTILES_bounding_volume_S2
  • 3DTILES_batch_table_hierarchy
  • 3DTILES_draco_point_compression

Some things to consider:

• See how ModelUtility.js handles glTF extensionsRequired and do something similar
• Also, we probably would need to update Model to include EXT_feature_metadata

[lilleyse]

Couple more notes for this:

• The new function should be Cesium3DTileset.checkSupportedExtensions modeled after ModelUtility.checkSupportedExtensions. It can be marked as @private in the doc.
• The function should be called in Cesium3DTileset.prototype.loadTileset
• Use oneTimeWarning to print the unsupported extension
• See https://github.com/CesiumGS/cesium/blob/master/Specs/Data/Cesium3DTiles/GltfContent/glTF/tileset.json for an example tileset that uses extensionsRequired.
• Make sure to add a test in Cesium3DTilesetSpec.js

[mramato]

Why would these be warnings instead of errors? Does a user have any way to handle this themselves so they could choose to not load the tileset or show a message to their users?

[lilleyse]

Hm, I suppose it should be a runtime error instead like what ModelUtility does.