add file picking for osx and ios and implement equirectangular import

engine/app.h

@@ -82,6 +82,7 @@ public:
     struct android_app* and_app;
     struct engine* and_engine;
 #endif
+    void pick_image(std::function<void(std::string path)> callback);
     void showKeyboard();
     void hideKeyboard();
     void initLog();