Ameliorate slow offline map rendering

[barefootguru]

Cachly is slow at rendering the offline maps, and Nik says this is hard to speed up.

If possible, I think it would help to:

1. prioritise rendering around the current location
2. when zooming in, don't clear the map — instead do a crude zoom on the current bitmap while the higher zoom renders in the background
3. display layers as they're rendered, rather than wait for them all to finish.  You could e.g. prioritise tracks over playgrounds.

Thanks!

[Nic Hubbard]

These things aren't easy to implement as the iOS MKMapView is what provides the tile x,y that need to be rendered, so there isn't a way to prioritize these.