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