Appends an extension to the list of extensions used by this GLTF file during serialization. If [param required] is true, the extension will also be added to the list of required extensions. Do not run this in [method GLTFDocumentExtension._export_post], as that stage is too late to add extensions. The final list is sorted alphabetically. Retrieves the array of texture samplers that are used by the textures contained in the GLTF. Sets the array of texture samplers that are used by the textures contained in the GLTF.