Quadkey tiles don't have a z0

[bcamper]

As far as I can tell, the quadkey tile scheme doesn't have a z0. The first quadkey tiles are at z1, and are 0, 1, 2, and 3. There's no all-encompassing earth tile. We'll need to figure out how to properly support this, other than limiting quadkey tile sources to a min display zoom of z1 (or, adding some kind of "under-zooming" support to load z1 quadkey tiles at z0, but this is a bit more complicated).

https://docs.microsoft.com/en-us/bingmaps/articles/bing-maps-tile-system