Add an option to drop a fraction of polygons by zoom.

Only warn once about polygon cleaning failures.

man/tippecanoe.1

@@ -138,6 +138,8 @@ compensate for the larger marker, or \-rf\fInumber\fP to allow at most \fInumber
 \-ao or \-\-reorder: Reorder features to put ones with the same properties in sequence, to try to get them to coalesce
 .IP \(bu 2
 \-al or \-\-drop\-lines: Let "dot" dropping at lower zooms apply to lines too
+.IP \(bu 2
+\-ap or \-\-drop\-polygons: Let "dot" dropping at lower zooms apply to polygons too
 .RE
 .SS Doing less
 .RS