update headers

ios/include/gltfio/transmission.spec

@@ -0,0 +1,22 @@
+ShadingModel=lit
+
+# Even though the transmission material actually has its
+# blend mode set to "masked", we want the glTF loader to
+# think that it is "opaque".
+BlendingMode=opaque
+
+Sheen=unsupported
+Volume=unsupported
+ClearCoat=unsupported
+
+VertexColors=optional
+BaseColorTexture=optional
+NormalTexture=optional
+OcclusionTexture=optional
+EmissiveTexture=optional
+MetallicRoughnessTexture=optional
+TextureTransforms=optional
+
+Transmission=optional
+Ior=optional
+TransmissionTexture=optional