tile 404 fallback mask level

[incanus]

Per @yhahn Mapnik has the concept of a fallback to a lower-numbered zoom level on blank tile 404s.

If we get a 404 around https://github.com/mapbox/llmr-native/blob/master/src/map/tile.cpp#L75-L85 we should reach back up to z8 similarly. I was getting bitten by this earlier and thought the tile server was messed up.

[jfirebaugh]

Fixed on the tile-serving side.