MSVC crash workaround

I do not know why, but the following line causes crash if assimp was compiled using MSVC with RelWithDebInfo configuration:
`std::transform(shading.begin(), shading.end(), shading.begin(), Assimp::ToLower<char>);`
replacing `shading.begin()` with `shading.data()` fixes this issue.

code/AssetLib/FBX/FBXMaterial.cpp

@@ -86,7 +86,7 @@ Material::Material(uint64_t id, const Element& element, const Document& doc, con
     std::string templateName;
 
     // lower-case shading because Blender (for example) writes "Phong"
-    std::transform(shading.begin(), shading.end(), shading.begin(), Assimp::ToLower<char>);
+    std::transform(shading.data(), shading.data() + shading.size(), shading.data(), Assimp::ToLower<char>);
     if(shading == "phong") {
         templateName = "Material.FbxSurfacePhong";
     }