minzoom & maxzoom may be incorrect using --merge

[warrenwyf]

The minzoom and maxzoom value of the metadata table may be incorrect, It looks like only the last zoom levels will be recorded, the correct one should be to perform a merger.

Maybe bounds has the same problem.

[systemed]

Ah, good point. bounds is explicitly handled at https://github.com/systemed/tilemaker/blob/master/src/tilemaker.cpp#L369-L372 but minzoom and maxzoom aren't - we should do that.