backend/{wayland,x11}: ensure buffers are released on shutdown
destroy_wl_buffer() is called from backend_destroy(). We need to ensure the wlr_buffer is unlocked when we're waiting for a wl_buffer.release event from the parent compositor.
destroy_wl_buffer() is called from backend_destroy(). We need to ensure the wlr_buffer is unlocked when we're waiting for a wl_buffer.release event from the parent compositor.