Fixed warnings when compiling for x64 on MSVC through VS 2017 v15.3.3.

code/ObjFileMtlImporter.cpp

@@ -304,7 +304,7 @@ void ObjFileMtlImporter::createMaterial()
         m_pModel->m_pCurrentMaterial = new ObjFile::Material();
         m_pModel->m_pCurrentMaterial->MaterialName.Set( name );
         if (m_pModel->m_pCurrentMesh) {
-            m_pModel->m_pCurrentMesh->m_uiMaterialIndex = m_pModel->m_MaterialLib.size() - 1;
+            m_pModel->m_pCurrentMesh->m_uiMaterialIndex = static_cast<unsigned int>(m_pModel->m_MaterialLib.size() - 1);
         }
         m_pModel->m_MaterialLib.push_back( name );
         m_pModel->m_MaterialMap[ name ] = m_pModel->m_pCurrentMaterial;