Pull Request for Zoom handling of (Overlay) MbTilesRenderer

As we have discussed, the zoom handling, which currently works for one level and is prepared for more levels but needs a review and further implementation to handle it correctly.

When during a request for an (Overlay) Mbtile no data is returned it modifies the request going up zoom levels until if finds data. 

This data is then processed to cut out the pixels of the desired area (here only one level down is working) and scales (working for multiple levels) them up depending on the zoomleveldifference until the desired tile size (256x256) is reached. 

Anyway, more than 2 or 3 levels should not be considered as then the result would be 16(2) or even 4 pixels which are scaled up.