Check AssImp shading model and use it to determine when to use Phong …

…instead of PBR

There are other AssImp shading models, but some of them aren't things the VSG's got a built-in shader for, and there's some dumbness with the available values as the list of options was originally taken from Blender back when Blender's shading models were total nonsense (e.g. Cook-Torrance was actually Blinn-Phong, Blinn-Phong had some parts of Cook-Torrance added etc.).
I've therefore ignored lots of them.

Previously, the value was put into a variable, but then never read, and that only happened on the Phong path, which was more-or-less unreachable as most assimp format loaders set AI_MATKEY_COLOR_SPECULAR for the Phong specular colour, so it wasn't a good signal to use to decide that PBR shaders should be used.

src/assimp/SceneConverter.cpp

@@ -533,7 +533,10 @@ void SceneConverter::convert(const aiMaterial* material, vsg::DescriptorConfigur
         defines.insert("VSG_TWO_SIDED_LIGHTING");
     }
 
-    if (getColor(material, AI_MATKEY_BASE_COLOR, pbr.baseColorFactor) || hasPbrSpecularGlossiness)
+    aiShadingMode shadingMode;
+    material->Get(AI_MATKEY_SHADING_MODEL, shadingMode);
+
+    if (!(shadingMode & aiShadingMode_Phong || shadingMode & aiShadingMode_Blinn) && (getColor(material, AI_MATKEY_BASE_COLOR, pbr.baseColorFactor) || hasPbrSpecularGlossiness))
     {
         // PBR path
         convertedMaterial.shaderSet = getOrCreatePbrShaderSet();
@@ -622,9 +625,6 @@ void SceneConverter::convert(const aiMaterial* material, vsg::DescriptorConfigur
         const auto emissiveResult = getColor(material, AI_MATKEY_COLOR_EMISSIVE, mat.emissive);
         const auto specularResult = getColor(material, AI_MATKEY_COLOR_SPECULAR, mat.specular);
 
-        aiShadingMode shadingModel = aiShadingMode_Phong;
-        material->Get(AI_MATKEY_SHADING_MODEL, shadingModel);
-
         unsigned int maxValue = 1;
         float strength = 1.0f;
         if (aiGetMaterialFloatArray(material, AI_MATKEY_SHININESS, &mat.shininess, &maxValue) == AI_SUCCESS)