backend-wayland: immediately unref the renderbuffer for use shm buffers

wayland_output_destroy_shm_buffers() is called immediately before
output_destroy() of the renderer is called. And for the pixman renderer all
renderbuffers must be destroyed before the output can be destroyed.
Also, weston_renderbuffer_unref() is not called when the buffer is released
because buffer->output is now NULL, so the renderbuffer would be leaked.

So just unref the renderbuffer immediately. Set it to NULL to avoid unreffing it
again should wayland_output_destroy_shm_buffers() be called again before the
buffer is released. This can happen during an xdg-shell resize.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>