Let userland retain the window backing store while drawing into it.

To start painting, call:
gui$get_window_backing_store()

Then finish up with:
gui$release_window_backing_store()

Process will retain the underlying GraphicsBitmap behind the scenes.
This fixes racing between the WindowServer and GUI clients.

This patch also adds a WSWindowLocker that is exactly what it sounds like.

Terminal/Terminal.cpp

@@ -26,6 +26,7 @@ void Terminal::create_window()
         exit(1);
     }
 
+    // NOTE: We never release the backing store.
     GUI_WindowBackingStoreInfo info;
     int rc = gui_get_window_backing_store(m_window_id, &info);
     if (rc < 0) {