Send dot-dropping through the same pipeline.

The first feature in a tile can never be dropped, since there is
no previous feature to attach its properties to.

Remove the previous special case that reset the dropping counter
at the first feature within each tile proper (as opposed to the
first feature in each tile, including its buffer, which is now
the one that is guaranteed to be preserved).

serial.hpp

@@ -66,6 +66,7 @@ struct serial_feature {
 	std::vector<std::string> full_keys{};
 	std::vector<serial_val> full_values{};
 	std::string layername = "";
+	bool dropped = false;
 };
 
 void serialize_feature(FILE *geomfile, serial_feature *sf, long long *geompos, const char *fname, long long wx, long long wy, bool include_minzoom);