Recent CesiumJS performance improvements for GeoJSON/MVT native support

[jo-chemla]

The recent CesiumJS releases from june 2026 mentions 2 interesting additions that might help TerriaJS/TerriaMap be more preformant for catalog item types geojson and mvt:

• Added GeoJsonPrimitive for loading GeoJSON directly into BufferPrimitiveCollections, bypassing the entity/DataSource layer for significantly improved performance with large datasets. #13505
• Added MVTDataProvider for loading Mapbox Vector Tiles (MVT) directly into CesiumJS as 3D Tiles. Supports per-feature styling via Cesium3DTileStyle, feature picking with metadata (getProperty), and automatic property table encoding via EXT_structural_metadata. #13404

[na9da]

Hi @jo-chemla, that's exciting!

I did test the new GeoJsonPrimitive and the performance is great, though we might need a couple of things to reach parity with our current geojson implementation in Terria:

1. Support for clamping/draping - I think currently the GeoJsonPrimitive features will float above or below the terrain
2. Strokes - if we want to keep the current visual style in Terria

I think both of these are tracked here in Cesium.

Maybe we could still add it behind a flag or something.