Custom metadata support

[romanov6416]

Hi!

First, thank you for the library development.
I want to use this lib in my python project. In addition to 3D geometry (draco mesh) I have to serialize/deserialize custom metadata attached to the mesh in the draco file. I see that current version of DracoPy doesn't support custom metadata. So I decided to make my own implementation of this in DracoPy.pyx file. And here I bumped into the problem:

I implemented recursive metadata struct.
in DracoPy.h:

  struct MetadataObject {
    std::unordered_map<std::string, std::vector<uint8_t>> entries;
    std::unordered_map<std::string, MetadataObject> sub_metadatas;
  };

And DracoPy.pxd

cdef extern from "DracoPy.h" namespace "DracoFunctions":
    cdef struct MetadataObject:
        unordered_map[string, vector[uint8_t]] entries
        unordered_map[string, MetadataObject] sub_metadatas

Also I added a function (to DracoPy.h) parsing draco metadata to MetadataObject and pass it into encode_mesh function.

And when I try to compile my solution I got cython compilation error:
RecursionError: maximum recursion depth exceeded while calling a Python object

I know declaring recursive structures is possible in Cython.
But it seems impossible to declare EXTERNAL recursive structures in Cython (I didn't find anything about it in google)

So my question is do you have ideas how to forward draco structure Metadata to Python?

[romanov6416]

Created PR #20 to support metadata

[william-silversmith]

Thanks for the PR romanov, I'm off until Friday, but I'll try to take a look then!