fix export animation frames and add export mp4

src/app.h

@@ -254,6 +254,7 @@ public:
     void dialog_browse();
     void dialog_export(std::string ext);
     void dialog_export_layers();
+    void dialog_export_anim_frames();
     void dialog_export_depth();
     void dialog_export_cube_faces();
     void dialog_layer_rename();