fix: replace runtime waitIdle calls with a deletion queue

destrum/src/App.cpp

@@ -331,8 +331,6 @@ if (!isRunning) break;
             }
         }
     }
-
-    gfxDevice.waitIdle();
 }
 
 void App::cleanup()
@@ -355,6 +353,10 @@ void App::cleanup()
         }
     };
 
+    // The one place we wait for the GPU on shutdown: the cleanup calls below destroy
+    // pipelines and other objects directly, which is only valid once nothing is in flight.
+    attempt([this] { gfxDevice.waitIdle(); });
+
     if (customInitStarted) {
         attempt([this] { customCleanup(); });
     }