Support for multiple terrain sources

Nobody has claimed this yet.

## Motivation

We have clients which do LIDAR scans & capture drone imagery of earthworks sites regularly (at least weekly). They will benefit greatly from being able to view 3d maps of terrain models derived from their LIDAR scans with the drone image draped over it.

Currently is appears that only a single terrain source can be specified, which means I either need to use our custom terrain model which is very limited in size, the mapbox whole earth terrain model or try and proxy the two together in order to serve them both under a single endpoint.

## Design Alternatives

Some sort of proxy setup should work, but we currently do not support any infrastructure for serving tilesets and would prefer to allow mapbox to provide this service for us.

## Design

it would be ideal if multiple terrain sources could be specified, which allows us to provide the mapbox DEM tileset as the 'base' terrain data set, then have our custom terrain models apply when available.

Start by reviewing how the current single terrain source is configured and consumed, then examine the terrain tile-loading path. Compare the requested base Mapbox DEM source and custom terrain models, including how availability and fallback should work. Done means multiple sources can be specified and custom terrain overrides the base data where available.

Written by the indexing model from the issue text.