Have nice fallbacks for tiles outside the poly passed and below maximum planet zoom level

When we create a ZIM of only a small area of the planet, we have:

• the whole planet up to level 6 (by default)
• the poly region up to level 14

While having the whole planet up to level 6 is convenient, it causes weird situations like below (where I made a ZIM of Corsica). Depending on the zoom level, we have blank areas which appears because corresponding tile did not make it to the ZIM. This also causes some 404 errors.

I don't think this is something to fix for 0.1 release, but still wanted to mention that limitation

Perfect:	Some tiles "missing":
Some tiles "missing":	Some tiles "missing":

I tried to investigate if an easy fix would be possible by continuing to use upper zoom data, but I failed to find proper definition for that to work.

Start by tracing how tiles are generated and looked up for the whole-planet level 6 data and the polygon region through level 14. Compare the missing-tile cases described for the Corsica ZIM and investigate whether upper-zoom data can provide a defined fallback. Done means tiles outside the polygon no longer appear blank or produce 404 errors at supported zoom levels.