demos: Handle paint events and call RedrawWindow to animate

This keeps MessageBoxs from validation on Windows from hanging the
demos, while allowing the demo to animate
diff --git a/demos/cube.c b/demos/cube.c
index a57bf4f..12de7f5 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -1814,7 +1814,7 @@
         break;
     case WM_PAINT:
         demo_run(&demo);
-        return 0;
+        break;
     default:
         break;
     }
@@ -2536,6 +2536,7 @@
             TranslateMessage(&msg); 
             DispatchMessage(&msg);
         }
+        RedrawWindow(demo.window, NULL, NULL, RDW_INTERNALPAINT);
     }
 
     demo_cleanup(&demo);