Currently these functions remove the node from the scene if the sibling
argument is the same node as the node. To prevent confusion when
misusing this API, assert that the nodes are distinct and document this.
This allows the compiler to error out if we haven't enumerated all
of the cases. This is useful to avoid a missing implementation when
adding a new node type.
This will allow more scene-graph extensions to be added without
cluttering wlr_scene.c, for instance for sub-surface handling and
wlr_output_layout integration.
