This allows compositors to easily add an xdg_surface to the
scene-graph while retaining the ability to unconstraint popups
and decide their final position.
Compositors can handle new popups with the wlr_xdg_shell.new_surface
event, get the parent scene-graph node via wlr_xdg_popup.parent.data,
create a new scene-graph node via wlr_scene_xdg_surface_tree_create,
and unconstraint the popup if they want to.
This is only called from one function.
To destroy the wlr_scene_subsurface_tree from elsewhere, callers
can destroy the scene-graph node returned by
wlr_scene_subsurface_tree_create instead (just like a compositor
would do). subsurface_tree_handle_surface_destroy does exactly this.
Inlining avoids calling subsurface_tree_destroy by mistake.
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.
