[wpigui] Don't recursively render frames in size callback (#3743)

WindowSizeCallback can sometimes be called while doing a render. If this occurs, imgui asserts. Avoid this case.

wpigui/src/main/native/include/wpigui_internal.h

@@ -37,6 +37,7 @@ struct Context : public SavedSettings {
   std::string title;
   int defaultWidth;
   int defaultHeight;
+  bool isPlatformRendering{false};
 
   GLFWwindow* window = nullptr;