add macOS implementation

macos/include/gltfio/volume.spec

@@ -0,0 +1,30 @@
+ShadingModel=lit
+
+# NOTE: this intentionally omits the blending mode spec because
+# Filament requires MASKED blending for KHR_materials_volume.
+
+# Core Features
+VertexColors=optional
+BaseColorTexture=optional
+NormalTexture=optional
+OcclusionTexture=optional
+EmissiveTexture=optional
+MetallicRoughnessTexture=optional
+TextureTransforms=optional
+
+# ClearCoat Features
+ClearCoat=unsupported
+
+# Sheen Features
+Sheen=unsupported
+SheenColorTexture=unsupported
+SheenRoughnessTexture=unsupported
+
+# Transmission and IOR Features
+Transmission=optional
+Ior=optional
+TransmissionTexture=optional
+
+# Volume Features
+Volume=optional
+VolumeThicknessTexture=optional