Mark Python palette as dirty after conversion to WEB palette

[radarhere]

Alternative to #9834

#9834 found that after converting an image to P or PA with the WEB palette, the Python palette was updated, but the C palette was not. It solves the problem by load()ing the Python palette into C at once (via putpalette()).

This PR instead just marks the Python palette as dirty, and the loading of the palette into C is lazily deferred until a future load() call.

[akx]

What's the reason for this None? 🤔

[radarhere]

It's returning the palette in the current palette mode, rather than the default of RGB - https://pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.getpalette

[akx]

Right, and .getpalette() calls load no matter what. 👍

[akx]

With this, my test from #9834 fails:

    def test_pa2p_truly_drops_alpha() -> None:
        im = Image.frombytes("P", (2, 1), bytes([0, 1])).convert("PA")
        im.putpalette(bytes([255, 0, 0, 7, 0, 255, 0, 9]), "RGBA")
        im.putalpha(Image.frombytes("L", (2, 1), bytes([240, 220])))
        assert im.get_flattened_data() == ((0, 240), (1, 220))  # Matches the alpha band
        im_p = im.convert("P")
        assert im_p.palette is not None
>       assert im_p.im.getpalettemode() == im_p.palette.mode == "RGB"
E       AssertionError: assert 'RGBA' == 'RGB'
E
E         - RGB
E         + RGBA
E         ?    +

[radarhere]

Yes, your test fails because you are accessing the core image directly - im_p.im. If you access it through any Python API, Pillow should call load() internally, and the core image will be updated.

[akx]

Ah yeah, that was it.

assert im_p.palette.mode == "RGB"
print(im_p.im.getpalettemode())
im_p.load()
print(im_p.im.getpalettemode())

prints out RGBA and RGB.

... Matter of separate discussion (and I'm surprised if no one's thought about it or discussed before), but I wonder if im.im should become a deprecated property accessor and im._im the actual field so it's clear you're touching the internal image...