add macOS implementation

macos/include/gltfio/lit_fade.spec

@@ -0,0 +1,32 @@
+# Required Aspects
+ShadingModel = ${SHADINGMODEL}
+BlendingMode = ${BLENDING}
+
+# Core Features
+VertexColors = optional
+BaseColorTexture = optional
+NormalTexture = optional
+OcclusionTexture = optional
+EmissiveTexture = optional
+MetallicRoughnessTexture = optional
+TextureTransforms = optional
+
+# ClearCoat Features
+ClearCoat = optional
+ClearCoatTexture = optional
+ClearCoatRoughnessTexture = optional
+ClearCoatNormalTexture = optional
+
+# Sheen Features
+Sheen = unsupported
+SheenColorTexture = unsupported
+SheenRoughnessTexture = unsupported
+
+# Transmission and IOR Features
+Transmission = unsupported
+Ior = unsupported
+TransmissionTexture = unsupported
+
+# Volume Features
+Volume = unsupported
+VolumeThicknessTexture = unsupported