Compositor: add BypassWindowManager hint to WindowFlags

BypassWindowManagerHint is already being used on the client-side, and it
would be beneficial for some compositors to see this server-side as well.

Change-Id: I4801a0c2f14f86fcd48cc49c3f2324af4216fb6f
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
Call sendRelease before unlockNativeBuffer

Unlock native buffer can call to destroy egl image.
If a wayland buffer is bound to the egl image and the buffer
resource is already destroyed by client, the buffer resouce
in compositor will be destroyed with destroying the egl image.
On sendRelease, m_buffer is null since destroy_listener_callback
is called by unlockNativeBuffer of HW integration.

Change-Id: I8fcaec12b4acab2c9d5fb2f2b4abc1e40654f014
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>