x11 drag: Fix mapped error

x11 drag: Fix mapped error

In the gtk-demo drag-and-drop demo i can't drag anything, all I get
is:

(gtk4-demo:358993): Gdk-CRITICAL **: 09:36:19.617: Surface 0x7e1bb0 has not been mapped in GdkSeatGrabPrepareFunc

This is because GdkX11Drag.ipc_surface is not considered mapped, even
though we called gdk_x11_surface_show() on it, because the
GDK_SURFACE_STATE_WITHDRAWN flag is still set.

I added calls to gdk_synthesize_surface_state() to match what
e.g. show_popup() and gdk_x11_toplevel_present() does.