Add support for reconstructing tiles from lower levels

The maximum cached level for a given <tileset> is specified
for each grid. E.g. to activate this, use

```
<tileset>
  <grid max-cached-zoom=12>WGS84</grid>
  ...
</tileset>
```

This will cause all tile above level 13 to be created
at request time by resampling from the tiles in level 12.