glTF 2.0: Assign default material to meshes with no material reference.

fixes  #1691

code/glTF2Importer.cpp

@@ -211,15 +211,9 @@ inline void SetMaterialTextureProperty(std::vector<int>& embeddedTexIdxs, Asset&
     }
 }
 
-void glTF2Importer::ImportMaterials(glTF2::Asset& r)
+static aiMaterial* ImportMaterial(std::vector<int>& embeddedTexIdxs, Asset& r, Material& mat)
 {
-    mScene->mNumMaterials = unsigned(r.materials.Size());
+    aiMaterial* aimat = new aiMaterial();
-    mScene->mMaterials = new aiMaterial*[mScene->mNumMaterials];
-
-    for (unsigned int i = 0; i < mScene->mNumMaterials; ++i) {
-        aiMaterial* aimat = mScene->mMaterials[i] = new aiMaterial();
-
-        Material& mat = r.materials[i];
 
    if (!mat.name.empty()) {
         aiString str(mat.name);
@@ -268,6 +262,21 @@ void glTF2Importer::ImportMaterials(glTF2::Asset& r)
 
         SetMaterialTextureProperty(embeddedTexIdxs, r, pbrSG.specularGlossinessTexture, aimat, aiTextureType_SPECULAR);
     }
+
+    return aimat;
+}
+
+void glTF2Importer::ImportMaterials(glTF2::Asset& r)
+{
+    const unsigned int numImportedMaterials = unsigned(r.materials.Size());
+    Material defaultMaterial;
+
+    mScene->mNumMaterials = numImportedMaterials + 1;
+    mScene->mMaterials = new aiMaterial*[mScene->mNumMaterials];
+    mScene->mMaterials[numImportedMaterials] = ImportMaterial(embeddedTexIdxs, r, defaultMaterial);
+
+    for (unsigned int i = 0; i < numImportedMaterials; ++i) {
+       mScene->mMaterials[i] = ImportMaterial(embeddedTexIdxs, r, r.materials[i]);
     }
 }
 
@@ -479,6 +488,10 @@ void glTF2Importer::ImportMeshes(glTF2::Asset& r)
             if (prim.material) {
                 aim->mMaterialIndex = prim.material.GetIndex();
             }
+            else {
+                aim->mMaterialIndex = mScene->mNumMaterials - 1;
+            }
+
         }
     }