Fixed transparency when saving P mode images to PDF

[radarhere]

Resolves #7312

The (last unresolved) image from the issue is

which saved as a PDF with main becomes

print(img.mode)
print(img.info)

gives

P
{'transparency': b'\x00PJ\xdb/@k\x0b\x1d\x18\xac~\x9a\x98\xd9m\xe2s\xde\xbc\xc2\x90\xb0\xde\xef\xe8\xd1\xed\xcf\xb5\xbd\xde\xf2\xd9\xf1\xeb\xf2\xeb\xee\xf8'}

This is not a single transparency index, but different levels of opacity for different palette entries.

To attach this to a P mode PDF image, this PR writes a secondary image, the image converted to just an alpha channel, as an "SMask".

https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf describes "SMask" under 8.9.5 'Image Dictionaries' as

A subsidiary image XObject defining a soft-mask image (see 11.6.5.3, "Soft-Mask Images") that shall be used as a source of mask shape or mask opacity values in the transparent imaging model.

With that, the PDF becomes

which can be converted back to a transparent PNG using ImageMagick's convert.