Lock around usages of imaging memory arenas

[lysnikolaou]

Related to #8199.

Did some testing with threads. Thread-sanitizer is also happy after these changes.

[lysnikolaou]

This is ready to be reviewed more throughly. All tests pass and tsan is happy.

[nulano]

All tests pass and tsan is happy.

Out of curiosity, how are you testing with tsan? I don't think we have it in CI (although perhaps we should add it!).

[lysnikolaou]

Out of curiosity, how are you testing with tsan? I don't think we have it in CI (although perhaps we should add it!).

I added a few very very basic threaded tests, built CPython and Pillow with tsan enabled and then ran the tests. It all takes forever, cause CPython with tsan is very slow, but it works.

If your question is how I build Pillow with tsan enabled, it is really easy:

CC="clang -fsanitize=thread" python -m pip install .

[hugovk]

If no other reviews or comments, I'll merge this tomorrow.

[hugovk]

Thanks @lysnikolaou!