Avoid unused object ids when saving PDF

[Rodrigo-Palma]

Changes proposed in this pull request:

• When saving a PDF, allocate each image object as it is written, instead of reserving them up front from the mode of the first frame.
• Add a test.

_save reserves an extra object ID for an SMask whenever im.mode == "P" and "transparency" in im.info, but it checks that once per image, against the current frame, and then reserves the same number for every frame. For a GIF with transparency, the first frame is P with transparency and the later frames are RGBA, so save_all=True reserves objects that are never written. They stay in the xref table with offset 0.

with Image.open("Tests/images/transparent_dispose.gif") as im:
    im.save("out.pdf", save_all=True)

On main, objects 9 and 10 have offset 0. PdfParser.read_indirect() raises "indirect object definition different than expected" for both, and qpdf --check warns "object has offset 0". With this change, every object reads back and qpdf --check is clean.

The extra ID was added in a70ea82 ("Write P transparency as SMask"). Letting write_obj(None, ...) allocate the ID means the count can no longer disagree with what is written.

[radarhere]

Thanks, nice catch. I've pushed a commit to add a comment and rename the test.