Support "Rehydrating" index

[kylebarron]

Motivation

The SnapFeatures constructor, which constructs the Rtree from the mesh positions, takes up the majority of the time.

This index could be useful in the future after the initial snapping of the basemap to the mesh.

For example, say a user is interested in uploading a GPS trace and wants to view it on top of the map, the process for displaying it might be:

• reproject WGS84 coordinates to web mercator
• cut line into web tiles, perhaps with the new utilities in math.gl
• for each tile, load a terrain mesh, and snap the line to it

Idea

Since Flatbush is based on typed arrays, "rehydrating" a flatbush index is easy and supported. https://github.com/mourner/flatbush#flatbushfromdata

This could make it easy to store the rtree for later use (as long as memory pressure isn't a concern)

• benchmark flatbush.from(data)

• expose an object from the SnapFeatures that includes

     indexData: Flatbush.data (TypedArray);
     triangles: FloatArray;
     bounds: [number, number, number, number] | null;

  (bounds might not be necessary). This would be an object of 3

[kylebarron]

Note that storing the index with the tile data would also help if I ever figured out the viewshed algorithm