Background tiles sometimes clear on zoom

[mourner]

Background tiles disappear If you zoom in and then zoom in more not waiting for the tiles to load.

[cgalli]

@mourner, I beleive this can be merged with the major bug #32.

[mourner]

It's a different issue because it's about scaled tiles that appear in background while the new ones load - it doesn't make the map unusable like in #32 where real tiles disappear, it's just a minor visual glitch.