Hiding QTWayland with wl_buffer NULL is not correct

Based on the commit reference, wl_buffer is attached as NULL to the wl_surface. wl_buffer NULL is not the correct way to use wayland.

Make it possible to hide windows
Change-Id: Iad80149278edd670fd55a3a6c1b4975ac81d7f59
Reviewed-by: Andy Nichols <andy.nichols@digia.com>

QtWayland::wl_surface::attach(0, 0, 0); is used to hide the surface. NULL surface is not valid as per wayland,

Even though the document says
>>If wl_surface.attach is sent with a NULL wl_buffer, the following wl_surface.commit will remove the surface content.

But causes unexpected behaviour that depends on compositor. xdg_surface will throw error and weston will get stuck.

For more information
https://www.mail-archive.com/wayland-devel@lists.freedesktop.org/msg23446.html