layer-shell: Require an initial commit

Created by: zzag

Currently, the compositor doesn't have enough information to decide if a given unmapped layer surface can be rearranged or rather configured.

In order to address the ambiguity, this change adjusts the protocol spec saying that every client must perform an initial commit without a buffer attached after creating and setting up a layer surface.

If the compositor knows that the initial state for a specified surface has not been committed yet, it can skip such a surface when re-arranging layer shell clients.