Add 3DTiles as source for terrain layer

[AbelVM]

As 3DTiles are gaining traction (Cesium, OGC, Google, MapTiler, etc.), MapLibre should be able to get the most of it.

Rendering a custom layer from a 3DTiles source is already solved through DeckGL, but maybe a killer feature would be to use those tiles as terrain source, with all the added functionality:

• zoom, pitch, and rotate user interactions would work as expected
• the tiles can be queried to retrieve the elevation, in the same ways terrain can be queried now (we could unify the API)
• we could seamlessly add any kind of 2D layers on top of 3D tiles
• the tiles requests are managed to avoid requesting far or occluded tiles, lowering the number of requested tiles for high pitch values

[HarelM]

Terrain has multiple manifestations: 3D terrain, hillshade, querying for elevation. I'm not sure how all of these work, but basically the DEMdata class is responsible to get elevation for a specific location. If you can show an example of how to use 3D tiles to get the elevation for a specific place then there might be a way to create a 3DtilesDEMSource similar to RasterDEMSource I guess. I have little knowledge in 3D tiles unfortunately...

[AbelVM]

As of today, CesiumJS API has 2 functions to do so that might inspire us:

• sampleHeight(position, objectsToExclude, width)
• sampleHeightMostDetailed(positions, objectsToExclude, width)

(links point to a starting point to dig in Cesium code :sweat_smile: )

First function is so reliable on zoom level, so we can just dismiss it. I've worked a bit on it, so I might keep on researching, but the current approach is based on deckGL as there is no native way to load 3DTIles yet