[WIP] Change vector density when zooming

This change sets a number of vectors to show when at minimum zoom, and quadruples the number at each increased zoom level, as long as there are enough vectors available.  In practice, this doesn't span the entire zoom range, so at higher levels the density will decrease.

Performance is an issue at higher zooms.  Perhaps something can be done to avoid spending time updating vectors outside the frame?

Closes #7.
A second approach would be to leave the full set of vectors in memory and tag them all with a maximum zoom level. Then split it into layers and add / remove them from the parent layer as needed. I'm not sure that it's any less complicated though.