We want to keep the wl_surface around, because surfaces create their
resources on construct and keep them until destroyed. See the HWND ond
Windows and the XWindow on X11.
This is relevant for graphics resources, where we want to have access
to the VkSurface and eglSurface while the GdkSurface is hidden.
We also want these surfaces to be permanent and not change during the
lifetime of the GdkSurface.
What we can - and must - destroy however are the xdg surfaces, because
those handle visibility on screen.
And we also need to ensure no buffer is attached, so that during the
next creation of the xdg surface we don't get a protocol error.
GdkDragSurface-backed widgets are not parented to an existing widget,
unlike popovers, and like toplevels. This means that there's nobody to
actively call gdk_drag_surface_present() to update the size, and
GdkDragSurface should do it on its own, just like GdkToplevel.
This commit implements this for the Wayland backend.
