Use low-resolution tiles when zooming in and the high res tiles are not yet available

[abey79]

Best effort background filling with lower-resolution tiles while hi-res tiles are being downloaded would greatly increase the user experience.

[podusowski]

There is #201, but I'd like to keep it opened, as it only tries one-level down. It's an improvement, but it can be better.

[abey79]

Just tested 0.26. That one level already makes a great deal of difference! So much less background flashing!