Release GIL during rank filters

[danking]

This doesn’t completely fix but is related to:
#6410

I noticed that MedianFilter doesn’t release the GIL which, for those of us not yet on free threaded Python, prevents us from fully utilizing all the cores.

I did use an LLM to do some research on pillow and to author these three lines but I would have written the exact same three. I simply don’t have a full sized keyboard at hand right now.

Thank you kindly for your time reviewing.

[akx]

LGTM (from the sidelines, as it were). (I should maybe take a look at other places where we could release the GIL more than we do now...)

#9744 already sped RankFilter up a little (unreleased as of yet).

My little eye also spies an opportunity to hoist those ysize and xsize (see #9788 etc.) for an additional speed boost, but that sounds out of scope for this PR.

[danking]

Hmm... That feels a bit like I might have uncovered some concurrency issue. Can't look carefully now but I can take a look in ~8 hours.

XFAIL Tests/test_imagedraw.py::test_line_h_s1_w2 - failing test
XFAIL Tests/test_image_resample.py::TestCoreResampleAlphaCorrect::test_levels_rgba - Current implementation isn't precise enough
XFAIL Tests/test_image_resample.py::TestCoreResampleAlphaCorrect::test_levels_la - Current implementation isn't precise enough
FAILED Tests/test_file_msp.py::test_open_windows_v2 - OSError: Corrupted MSP file in row 233

[akx]

Out of curiosity, @danking: what size are the MedianFilters you generally run (in, I assume, production code)?

[danking]

You are too generous! This is just for a silly project of mine. The files are all very tiny, on the order of tens to hundreds of KB. The filters are 3x3 or 5x5. You shouldn't take anything I'm doing here as grounded in any serious production work.

[akx]

Silly projects are the best projects! But okay, I was just thinking if it'd maybe be worth it to special-case the 3x3 median case. I'd imagine it's a fairly common one.

[danking]

Haha, agreed. I think you can accelerate that case. I have some ideas. I'll toss a PR up if one of them turns out to be good.