* Reduce maxzooms being guessed a little:
* Use 1.5 standard deviations, not 2, as the minimum distinguishable
* Give overlapping polygons and linestrings more distinct indices
* Add another drop rate guessing options, from the same metrics -zg uses
* Guard against using -rp without -zg
(The feature count when filtering will be the sum of features
across tiles instead of filters from the original input, since
the filter reader doesn't know what the original input feature
set was.)
Erica Fischer