mapt build --single: determine a policy for overzooming

[cldellow]

when developing, it can be handy to have an expensive slice configured with a lower maxzoom than the rest of your map.

For example, if you had two slices:

• ocean (pmtiles maxzoom 10, ocean layer maxzoom 10)
• land (pmtiles maxzoom 14, ways layer maxzoom 14)

You only generate ocean tiles up to z10, and then MapLibre overzooms when rendering the map, so the map appears visually correct (if a bit rough around the edges, due to overzooming a simplified geometry).

But when you merge, we have to pick a single maxzoom for the merged pmtiles. We pick the highest one, i.e. z14. But now your z10 ocean layer is no longer eligible for overzooming.

Should we do anything special here? For now, I think leave it as a footgun, and expect users to do whatever makes sense for their case. Hopefully real world use will inform what the policy, if any, should be.