When opening a Godot 4.5 project in 4.6 and saving scenes with editable
children of imported scenes (e.g., .blend files), all animation data was
incorrectly being saved to the .tscn file.
This happened because the AnimationLibrary serialization format changed
between 4.5 and 4.6:
- 4.5: 'libraries' as a single Dictionary property
- 4.6: 'libraries/<name>' as separate properties per library
When the scene packer looked for 'libraries/<name>' in old imported
scenes, it didn't find them (only 'libraries' existed), causing all
animation data to appear as 'overridden' and be saved.
This fix adds backwards compatibility in SceneState::get_property_value()
to check for the old Dictionary format when looking up 'libraries/<name>'
properties. It only applies to nodes that inherit from AnimationMixer,
allowing property comparison to work correctly and preventing animation
data from being duplicated into .tscn files.
Fixes#113037
