WindowServer: Fix drawing wallpaper on additional screens

We need to translate the source rectangle relative to the screen
location when blitting from the wallpaper bitmap.

Userland/Services/WindowServer/Compositor.cpp

@@ -320,7 +320,7 @@ void Compositor::compose()
  painter.draw_tiled_bitmap(rect, *m_wallpaper);
  } else if (m_wallpaper_mode == WallpaperMode::Stretch) {
  VERIFY(screen.compositor_screen_data().m_wallpaper_bitmap);
- painter.blit(rect.location(), *screen.compositor_screen_data().m_wallpaper_bitmap, rect);
+ painter.blit(rect.location(), *screen.compositor_screen_data().m_wallpaper_bitmap, rect.translated(-screen.location()));
  } else {
  VERIFY_NOT_REACHED();
  }