Reduce maxzooms being guessed a little: (#2)

* 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

man/tippecanoe.1

@@ -559,6 +559,8 @@ the same layer, enclose them in an \fB\fCall\fR expression so they will all be e
 If you use \fB\fC\-rg\fR, it will guess a drop rate that will keep at most 50,000 features in the densest tile.
 You can also specify a marker\-width with \fB\fC\-rg\fR\fIwidth\fP to allow fewer features in the densest tile to
 compensate for the larger marker, or \fB\fC\-rf\fR\fInumber\fP to allow at most \fInumber\fP features in the densest tile.
+If you use \fB\fC\-rp\fR with \fB\fC\-zg\fR or \fB\fC\-\-smallest\-maximum\-zoom\-guess\fR it will choose a drop rate from the same
+distance\-between\-features metrics as are used to choose the maxzoom.
 .IP \(bu 2
 \fB\fC\-B\fR \fIzoom\fP or \fB\fC\-\-base\-zoom=\fR\fIzoom\fP: Base zoom, the level at and above which all points are included in the tiles (default maxzoom).
 If you use \fB\fC\-Bg\fR, it will guess a zoom level that will keep at most 50,000 features in the densest tile.