Unlike all other ExtensionType_new functions, PyGlyph_new is not a
tp_new (https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_new)
but simply a plain C function used to construct a struct from the fields
of a FT2Font. Rename it to avoid confusion, and make it take a single
FT2Font as argument, too (with some additional fixes for
const-correctness).

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).