Factor out the GeoJSON parsing loop from feature serialization

geojson-loop.hpp

@@ -0,0 +1,11 @@
+#include <string>
+#include "jsonpull/jsonpull.h"
+
+struct json_feature_action {
+	std::string fname;
+
+	virtual int add_feature(json_object *geometry, json_object *properties, json_object *id, json_object *tippecanoe, json_object *feature) = 0;
+	virtual void check_crs(json_object *j) = 0;
+};
+
+void parse_json(json_feature_action *action, json_pull *jp);