message: gltfpack: Fix multi-material .obj mesh parsing
  When reading index data from .obj structures, offset increases linearly
  in the group regardless of the material index.
diff: @@ -259,6 +259,8 @@ void parseMeshesObj(fastObjMesh* obj, std::vector<Mesh>& meshes)
  for (unsigned int gi = 0; gi < obj->group_count; ++gi)
  {
  + size_t group_offset = 0;
  +
  for (unsigned int fi = 0; fi < obj->groups[gi].face_count; ++fi)
  {
  unsigned int mi = obj->groups[gi].materials[fi];
  @@ -269,7 +271,7 @@ void parseMeshesObj(fastObjMesh* obj, std::vector<Mesh>& meshes)
  for (unsigned int vi = 0; vi < obj->groups[gi].vertices[fi]; ++vi)
  {
  - fastObjIndex ii = obj->groups[gi].indices[vo + vi];
  + fastObjIndex ii = obj->groups[gi].indices[group_offset + vi];
  Attr p = { obj->positions[ii.p * 3 + 0], obj->positions[ii.p * 3 + 1], obj->positions[ii.p * 3 + 2] };
  Attr n = { obj->normals[ii.n * 3 + 0], obj->normals[ii.n * 3 + 1], obj->normals[ii.n * 3 + 2] };
  @@ -289,6 +291,7 @@ void parseMeshesObj(fastObjMesh* obj, std::vector<Mesh>& meshes)
  vertex_offset[mi] += obj->groups[gi].vertices[fi];
  index_offset[mi] += (obj->groups[gi].vertices[fi] - 2) * 3;
  + group_offset += obj->groups[gi].vertices[fi];
  }
  }
  }
  