Ensure Image and core palettes are in sync after PA conversion (alphas dropped)

Author: akx

Tests/test_image_convert.py

@@ -290,7 +290,6 @@ def test_p2pa_palette() -> None:
     assert im_pa.getpalette() == im.getpalette()
 
 
-@pytest.mark.xfail(reason="zombie colors from palette shouldn't reappear")
 def test_pa2p_truly_drops_alpha() -> None:
     im = Image.new("PA", (2, 1))
     # PA mode with an RGBA palette (very faint alphas here).

src/PIL/Image.py

@@ -1245,6 +1245,8 @@ def convert_transparency(
             from . import ImagePalette
 
             new_im.palette = ImagePalette.ImagePalette("RGB", im.getpalette("RGB"))
+            new_im.palette.dirty = True
+            new_im.load()
         if delete_trns:
             # crash fail if we leave a bytes transparency in an rgb/l mode.
             del new_im.info["transparency"]