pixman-renderer: Unref the pixman image when wl_shm_buffer is gone

Even if the weston_buffer_reference is still alive in situations like
when we have closing animations, the underyling buffer (wl_shm_buffer)
is no longer available. Call the appropriate destroy handler to
invalidate the pixman image and avoid touch the shm_buffer.

Fixes: #613

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>