Always allow zooming out enough to see full coverage area

[ajashton]

Navigating a tileset is difficult when the coverage area of the minimum zoom level is significantly larger than what fits on the screen at one time. The use should be able to zoom out further so they can zoom in again at a different location.

We need something to display to help the user navigate when they are seeing a non-existent zoom level.

• A built-in/default reference layer (see provide an optional built-in background reference layer #6)
• A representation of the tile coverage area - eg draw the extents of all the tiles present in the minzoom. We would need to start simplifying & aggregating this information once you have zoomed out by more than about 4 or 5 zoom levels.