shutdown based on platform, update accessors for editors

src/app/core/controller.cc

@@ -358,8 +358,19 @@ void Controller::OnExit() {
     SDL_CloseAudioDevice(audio_device_);
     delete audio_buffer_;
   }
-  ImGui_ImplSDLRenderer2_Shutdown();
-  ImGui_ImplSDL2_Shutdown();
+  switch (platform_) {
+    case Platform::kMacOS:
+    case Platform::kWindows:
+    case Platform::kLinux:
+      ImGui_ImplSDLRenderer2_Shutdown();
+      ImGui_ImplSDL2_Shutdown();
+      break;
+    case Platform::kiOS:
+          // Deferred
+      break;
+    default:
+      break;
+  }
   ImGui::DestroyContext();
   SDL_Quit();
 }

src/app/core/controller.h

@@ -40,6 +40,8 @@ class Controller : public ExperimentFlags {
   void DoRender() const;
   void OnExit();
 
+  auto master_editor() -> editor::MasterEditor & { return master_editor_; }
+
  private:
   struct sdl_deleter {
     void operator()(SDL_Window *p) const {