MbTiles(Overlay) zoom interpolation 

Ok, finally a clean pull request.

This will, when no (overlay) MbTiles data is found, modify the database request until it finds a tile.

If no data is found it gives up and as before no tile is returned.

If found, the zoom level difference is checked.

Edit. This will work for higher zoom level differences now, but it is still limited to one as we discussed. 

According to that difference the data (pixels) are cut out of the available tile, scaled up and returned as a tile of 256x256 pixel.

Anyway, as for a difference of 2 or 3 only 16 resp. 4 pixels would be upscaled, a limit of 2 different levels would be reasonable, according to me. Or leave it as it is, as one level is working out.

For zoom level differences larger than one, as before, no tile is returned, hence no MbTiles overlay.