SpecularFactor import and export improvements

The changes here (which only apply to reading from or writing to pbrSpecularGlossiness) will:

- store and read specular color on `AI_MATKEY_COLOR_SPECULAR ` rather than `AI_MATKEY_GLTF_PBRSPECULARGLOSSINESS_SPECULAR_FACTOR`
- store and read specular texture from `aiTextureType_SPECULAR` rather than `AI_MATKEY_GLTF_PBRSPECULARGLOSSINESS_SPECULARGLOSSINESS_TEXTURE`. Even though pbrSG’s specularGlossiness texture uses the alpha channel for glossiness, it will still work well enough with just the RGB channels of the image

code/glTF2Exporter.cpp

@@ -470,11 +470,11 @@ void glTF2Exporter::ExportMaterials()
 
             PbrSpecularGlossiness pbrSG;
 
-            GetMatColor(mat, pbrSG.specularFactor, AI_MATKEY_GLTF_PBRSPECULARGLOSSINESS_SPECULAR_FACTOR);
             mat->Get(AI_MATKEY_GLTF_PBRSPECULARGLOSSINESS_GLOSSINESS_FACTOR, pbrSG.glossinessFactor);
-            GetMatTex(mat, pbrSG.specularGlossinessTexture, AI_MATKEY_GLTF_PBRSPECULARGLOSSINESS_SPECULARGLOSSINESS_TEXTURE);
             GetMatColor(mat, pbrSG.diffuseFactor, AI_MATKEY_COLOR_DIFFUSE);
+            GetMatColor(mat, pbrSG.specularFactor, AI_MATKEY_COLOR_SPECULAR);
             GetMatTex(mat, pbrSG.diffuseTexture, aiTextureType_DIFFUSE);
+            GetMatTex(mat, pbrSG.specularGlossinessTexture, aiTextureType_SPECULAR);
 
             m->pbrSpecularGlossiness = Nullable<PbrSpecularGlossiness>(pbrSG);
         }