Add support for mesh terrain tilesets like those produced by STK Terrain Server

- Extended `CesiumTerrainProvider` to work with both the original heightmap-based terrain tiles and also the new mesh-based ones.
- Added `QuantizedMeshTerrainData` which is the mesh analog of `HeightmapTerrainData`.  It holds `u`, `v`, and `height` quantized as 16-bit integers.
- Fixed some bugs with terrain providers that aren't "always ready" like `CesiumTerrainProvider` used to be.
- Added some additional features to TerrainInspector.
- Removed the `latitudeFactor` entirely for terrain, and included it for imagery only when the imagery has a `WebMercatorTilingScheme`.
- Added two new web workers: `createVerticesFromQuantizedMesh` and `upsampleQuantizedTerrainMesh`.
- Added `makeRelativeUrlAbsolute` to build an absolute URL from a base URL and a relative URL.
- Added `Intersections2D` class containing operations on 2D triangles.