message: Fix OBJ importer
  If only some meshes have shading normals or texture coordinates then
  invalid normals / tcs were created for meshes that didn't have them.
diff: @@ -230,7 +230,8 @@ namespace ospray {
  attrib.vertices[idx2.vertex_index * 3 + 1],
  attrib.vertices[idx2.vertex_index * 3 + 2]));
  - if (!attrib.normals.empty()) {
  + // TODO create missing normals&texcoords if only some faces have them
  + if (!attrib.normals.empty() && idx0.normal_index != -1) {
  vn->push_back(vec3f(attrib.normals[idx0.normal_index * 3 + 0],
  attrib.normals[idx0.normal_index * 3 + 1],
  attrib.normals[idx0.normal_index * 3 + 2]));
  @@ -242,7 +243,7 @@ namespace ospray {
  attrib.normals[idx2.normal_index * 3 + 2]));
  }
  - if (!attrib.texcoords.empty()) {
  + if (!attrib.texcoords.empty() && idx0.texcoord_index != -1) {
  vt->push_back(vec2f(attrib.texcoords[idx0.texcoord_index * 2 + 0],
  attrib.texcoords[idx0.texcoord_index * 2 + 1]));
  vt->push_back(vec2f(attrib.texcoords[idx1.texcoord_index * 2 + 0],
  