Add default: break; to switches on SDL_Event::type

This is just a nice-to-have, as `SDL_Event::type` in an `Uint32`, so there are no
compiler errors about not handling all `SDL_EventType` enum values.

test/testoverlay.c

@@ -282,6 +282,8 @@ static void loop(void)
             if (event.key.keysym.sym != SDLK_ESCAPE) {
                 break;
             }
+        default:
+            break;
         }
     }