Clean: Fixing depr warnings in Assets/Editor
- Removed obsolete BuildOptions usage (they do nothing on native side)
- Using explicit EditorGUILayout.ObjectField with AllowSceneUsage parameter overload
- More PrefabUtility conversion
- TextureImporterFormat.AutomaticTruecolor -> TextureImporterCompression.Uncompressed
12 warnings to go.
Tests: none, trivial changes