WindowServer+LibGUI: Send the window size along with Paint server messages.

This way GWindow doesn't need to do synchronous IPC to fetch the appropriate
size for the window's backing store. This is mostly only relevant during
live resize.

WindowServer/WSClientConnection.cpp

@@ -349,10 +349,12 @@ void WSClientConnection::handle_request(WSAPIInvalidateRectRequest& request)
         post_error("Bad window ID");
         return;
     }
+    auto& window = *(*it).value;
     WSAPI_ServerMessage response;
     response.type = WSAPI_ServerMessage::Type::Paint;
     response.window_id = window_id;
     response.paint.rect = request.rect();
+    response.paint.window_size = window.size();
     post_message(response);
 }