[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.
It's more machinery (and more garbage collection) but taking a resizable subset of the vectors based on zoom level and then only updating what's visible seems ok, see #51.