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

Only warn once about polygon cleaning failures.

tile.h

@@ -46,6 +46,8 @@ static int additional_options[] = {
 	A_REORDER,
 #define A_LINE_DROP ((int) 'l')
 	A_LINE_DROP,
+#define A_POLYGON_DROP ((int) 'p')
+	A_POLYGON_DROP,
 #define A_PREFER_RADIX_SORT ((int) 'r')
 	A_PREFER_RADIX_SORT,
 };