Zoom level not used for validity range

If you encode a vector with a specific zoom range, let's say -z 4 -Z 6, tl info should return "minzoom":4,"maxzoom":6, not "minzoom":0,"maxzoom":22, as it does now.

May be adding some parameters like --no_overzooming --no_underzooming.

But you can solve that thanks to the -i parameter.

However tessera displays for a given level all the layers at this level on a higher level.
