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.

LibGUI/GWindow.cpp

@@ -168,9 +168,9 @@ void GWindow::event(GEvent& event)
         auto rect = paint_event.rect();
         bool created_new_backing_store = !m_backing;
         if (!m_backing) {
-            // NOTE: size() may change at any time since it's synchronously retrieved from the WindowServer.
             ASSERT(GEventLoop::main().server_pid());
-            Size new_backing_store_size = size();
+            ASSERT(!paint_event.window_size().is_empty());
+            Size new_backing_store_size = paint_event.window_size();
             size_t size_in_bytes = new_backing_store_size.area() * sizeof(RGBA32);
             void* buffer;
             int shared_buffer_id = create_shared_buffer(GEventLoop::main().server_pid(), size_in_bytes, (void**)&buffer);