feat(core): Add public stringifyGeoJSON to mbgl::style::conversion (#4345)

Co-authored-by: Bart Louwers <bart@emeel.net>

include/mbgl/style/conversion/geojson.hpp

@@ -4,6 +4,7 @@
 #include <mbgl/style/conversion.hpp>
 
 #include <optional>
+#include <string>
 
 namespace mbgl {
 namespace style {
@@ -12,6 +13,12 @@ namespace conversion {
 // Workaround until https://github.com/mapbox/mapbox-gl-native/issues/5623 is done.
 std::optional<GeoJSON> parseGeoJSON(const std::string&, Error&);
 
+// Serialize a GeoJSON value back to a JSON string. This is the public
+// counterpart to parseGeoJSON: consumers that link the core through the
+// amalgam (which exports only mbgl::* symbols) cannot reach the bundled
+// mapbox::geojson::stringify directly.
+std::string stringifyGeoJSON(const GeoJSON&);
+
 template <>
 struct Converter<GeoJSON> {
 public:

src/mbgl/style/conversion/geojson.cpp

@@ -2,6 +2,8 @@
 #include <mbgl/style/conversion/json.hpp>
 #include <mbgl/style/conversion_impl.hpp>
 
+#include <mapbox/geojson.hpp>
+
 namespace mbgl {
 namespace style {
 namespace conversion {
@@ -14,6 +16,10 @@ std::optional<GeoJSON> parseGeoJSON(const std::string& value, Error& error) {
     return convertJSON<GeoJSON>(value, error);
 }
 
+std::string stringifyGeoJSON(const GeoJSON& geojson) {
+    return mapbox::geojson::stringify(geojson);
+}
+
 } // namespace conversion
 } // namespace style
 } // namespace mbgl