common saver: change to instance based for future sync/async behavior.

reserve sync() method TODO

inc/thorvg.h

@@ -1402,9 +1402,11 @@ public:
      * @retval Result::MemoryCorruption When casting in the internal function implementation failed.
      * @retval Result::Unknown Others.
      *
-     * @BETA_API
      */
-    static Result save(std::unique_ptr<Paint> paint, const std::string& path) noexcept;
+    Result save(std::unique_ptr<Paint> paint, const std::string& path) noexcept;
+    Result sync() noexcept;
+
+    static std::unique_ptr<Saver> gen() noexcept;
 
     _TVG_DECLARE_PRIVATE(Saver);
 };

src/lib/tvgSaver.cpp

@@ -59,3 +59,16 @@ Result Saver::save(std::unique_ptr<Paint> paint, const std::string& path) noexce
     delete p;
     return Result::Unknown;
 }
+
+
+Result Saver::sync() noexcept
+{
+    //TODO:
+    return Result::Success;
+}
+
+
+unique_ptr<Saver> Saver::gen() noexcept
+{
+    return unique_ptr<Saver>(new Saver);
+}