0.14.0 release

[lilleyse]

Changes:

• Added support for EXT_structural_metadata. Property values can be accessed in material graph with the cesium_property nodes.
• Added support for EXT_mesh_features. Feature ID values can be accessed in material graph with the cesium_feature_id_int node.
• Added support for custom glTF vertex attributes. Attribute values can be accessed in material graph with the data_lookup nodes.
• Added support for changing a tileset's imagery layer dynamically in material graph.