[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.
By keeping a reference to the previous zoom level, we should be able to avoid iterating over *all* the vectors when it changes and avoid touching *any* of them when it doesn't.

I'll try this now.