3D Tiles Next roadmap

[lilleyse]

Roadmap issue to keep track of 3D Tiles Next extensions and their implementation status in Cesium Native.

See 3D Tiles Next README for more background on these extensions.

3D Tiles Next extensions:

• [x] 3DTILES_content_gltf
• [ ] ~3DTILES_metadata~
• [x] EXT_mesh_features - Successor to EXT_feature_metadata (#281)
• [x] 3DTILES_implicit_tiling - https://github.com/CesiumGS/cesium-native/issues/353
• [x] 3DTILES_bounding_volume_S2 - https://github.com/CesiumGS/cesium-native/issues/364
• [ ] 3DTILES_multiple_contents

Other useful glTF extensions

• [ ] EXT_mesh_gpu_instancing - https://github.com/CesiumGS/cesium-native/pull/290 (related)
• [x] KHR_texture_basisu
• [x] EXT_meshopt_compression
• [x] KHR_mesh_quantization

The initial 3D Tiles Next datasets we want to show are OWT and CDB. The OWT base globe uses nearly all the 3D Tiles Next extensions. CDB data can be converted to 3D Tiles Next with the --3d-tiles-next flag once https://github.com/CesiumGS/cdb-to-3dtiles/pull/58 is merged. Datasets like Yemen have raster materials that are converted to feature ID textures.

OWT base globe	CDB with feature ID textures

[lilleyse]

Most of this has been implemented already, with the exception of multiple contents and EXT_mesh_gpu_instancing (which is being tracked here).