fix: more robust mbtiles handling
fix: more robust mbtiles handling

If you construct mbtiles from a file system images pyramid
it may be missing name, so check instead something meaningful like
bounds to check if the file is actually a valid mbtiles file.
Fixes #300.

webmasterkai/signalk-server-node,sbender9/signalk-server-node,SignalK/signalk-server-node,sbender9/signalk-server-node,SignalK/signalk-server-node,webmasterkai/signalk-server-node,webmasterkai/signalk-server-node,SignalK/signalk-server-node,SignalK/signalk-server-node,sbender9/signalk-server-node