PR Summary

Adding on to the changes from the original PR: #20740, this PR modifies the layout of FT2Font to allow multiple fonts to work with the PDF backend.

Agg backend is simpler to work with, since things like kerning are set within FT2Font, and all we really needed was a bitmap from it. However, for the PDF backend, we need to extract each glyph from a (possibly) parent FT2Font.

To tackle this, I've implemented a caching layer for the parent object, allowing us to directly get the relevant FT2Font which has a certain glyph. Bits and pieces behind how PDF backend handles fonts is also modified.

With this, we can successfully generate Type 3 and Type 42 subsetted multi-font PDFs:
type3_fallback.pdf
type42_fallback.pdf

Test Script:

import matplotlib.pyplot as plt

FONTTYPE = 42    # or 3

plt.rcParams['font.family'] = ['Authentic', 'SimHei']
plt.rcParams['pdf.fonttype'] = FONTTYPE
plt.rcParams['font.size'] = 30
plt.rcParams['pdf.compression'] = 0

plt.figtext(0.18, 0.45, "There are 多个汉字 in between!")
plt.savefig(f"type{FONTTYPE}_fallback.pdf")

Builds on: #20740, Fixes #18883, #15260

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• 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).