WindowServer: Give windows a "background color" to use for missing parts.

When resizing a window, we often end up having to paint some part of it
without coverage in the current backing store. This patch makes those cases
look nicer by having a fallback background color for each window, passed
along with the CreateWindow client message.

Applications/Terminal/main.cpp

@@ -88,6 +88,7 @@ int main(int argc, char** argv)
     make_shell(ptm_fd);
 
     auto* window = new GWindow;
+    window->set_background_color(Color::Black);
     window->set_double_buffering_enabled(false);
     window->set_should_exit_event_loop_on_close(true);