Add needs_composition APIs
Right now, compositors are expected to query liftoff_layer_get_plane_id to decide whether a layer needs to be composited.
However there are cases where a layer has no plane, but still doesn't need composition (e.g. when FB_ID is zero or when alpha is zero, in the future when the layer is out of CRTC bounds etc).
Add a new API to make users' life simpler.