Better support for non-tiled data

[WilliamVenner]

Motivation

Mapbox is currently actively hostile to scenarios where map data is not tiled.

I have two images: the map, and the heightmap.

Working with this has been an extreme pain. Mapbox assumes almost everywhere that you just use tiles.

Please implement better support for single-image sources, especially in the case of heightmaps, as single-image maps already work OK.

[WilliamVenner]

I managed to discover that my main issue making this all a pain was the fact that I used an image source for my map layer, rather than a raster source with a single tile. After using a raster source with a single tile (like I was doing with the heightmap's raster-dem source) most of my issues magically poofed away.