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

```xml
<tileset>
  <grid max-cached-zoom="12">WGS84</grid>
  <!-- ... -->
</tileset>
```

This will cause all tiles above level 13 to be created
at request time by resampling from the tiles in level 12.