xcb: Export RootWindow to QXcbNativeInterface::nativeResourceForScreen()

Change-Id: I0ad74de8aef7d8a3cb707b61438096c67cd7626e
Reviewed-by: David Faure <david.faure@kdab.com>

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -248,6 +248,9 @@ void *QXcbNativeInterface::nativeResourceForScreen(const QByteArray &resourceStr
     case NoFontHinting:
         result = xcbScreen->noFontHinting() ? this : 0; //qboolptr...
         break;
+    case RootWindow:
+        result = reinterpret_cast<void *>(xcbScreen->root());
+        break;
     default:
         break;
     }