wlr_scene: Only allow parenting on a wlr_scene_tree

Requires: !3497 (merged)

Breaking changes:

• The root node is no longer a special case. The WLR_SCENE_NODE_ROOT enum is now removed and the root is just a glorified wlr_scene_tree
• The root node no longer has a reference to a wlr_scene_node but instead a wlr_scene_tree
• wlr_scene_node_state has been inlined into wlr_scene_node
• wlr_scene_tree now has the children linked list, not the wlr_scene_node
• All reparenting and scene node creation will now only accept a wlr_scene_tree

Sway patch: https://github.com/Nefsen402/sway/commit/b7f0b4adc4423a46363c69d25a04b021aa8abf81