Fix race condition: serialize Pango/Cairo calls for thread safety

[SoldierSacha]

Summary

• Adds a module-level threading.Lock to serialize all calls into Pango/Cairo
  and font registration, fixing a race condition caused by concurrent access to
  global state (the default PangoCairoFontMap, Fontconfig config, and the
  shared registered_fonts set).
• Replaces star imports (from .cmanimpango import *, etc.) with explicit
  imports and a _synchronized decorator, making the public API surface clear
  and the thread-safety wrapping declarative.

Problem

Pango and Cairo use process-global state internally (the default font map,
Fontconfig configuration). When multiple threads call text2svg,
MarkupUtils.text2svg, or font registration functions concurrently, this
leads to corrupted state, segfaults, or incorrect rendering — particularly
in tools like Manim that render scenes in parallel.

Approach

A single threading.Lock (_pango_lock) in __init__.py wraps every
public function that touches Pango/Cairo or the font registry. A small
_synchronized decorator (using functools.wraps) keeps the code DRY
and preserves function metadata (__name__, __doc__, etc.).

Only manimpango/__init__.py is changed — no modifications to the Cython
layer or register_font.py.

[naveen521kk]

Thanks for your contribution. I took a look around this, and it looks like a workaround solution instead of fixing the actual issue. Instead, we have to change the Cython implementation to implement locks, if required, accordingly.

Do you have a Minimal Reproducible Example where we can reproduce this issue? We need it to add it to the test suite.

[SoldierSacha]

Thanks for your contribution. I took a look around this, and it looks like a workaround solution instead of fixing the actual issue. Instead, we have to change the Cython implementation to implement locks, if required, accordingly.

Do you have a Minimal Reproducible Example where we can reproduce this issue? We need it to add it to the test suite.

No problem! I guess you could consider it a workaround solution in that sense, since the problem does technically come from the Cython.

Unfortunately I don't have a minimal example. If that's necessary, I guess you can close the PR. I just wanted to bring this to attention.