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/testoffscreen.c

@@ -74,6 +74,8 @@ static void loop(void)
         case SDL_EVENT_QUIT:
             done = SDL_TRUE;
             break;
+        default:
+            break;
         }
     }