Only write GIF comments at the beginning of the file

[raygard]

Helps #6207

Place GIF comment after Global Color table. Should go after "NETSCAPE" looping extension after pull #6211.

Fixes #6299.

Changes proposed in this pull request:

• Move writing of comment block from _write_local_header() to _get_global_header()
• Add a test test_write_comment() to validate that a comment is written to the first frame only
• Add a test test_write_no_comment() to validate that an empty comment="" arg to .save leaves no comments in saved file
• Tests use image file multiple_comments.gif added in PR Separate multiple GIF comment blocks with newlines #6294 but could use any animated GIF instead — CHANGED to dispose_prev.gif’

The change here to _get_global_header() overlaps the change in PR #6211. Please be sure the merge is done so that the if "comment" in info and len(info["comment"]): code in this PR is placed after the if "loop" in info: code in PR #6211. The comment must come after the "NETSCAPE2.0" block.

[raygard]

I updated the tests to use dispose_prev.gif so that the automated tests would complete.

[raygard]

@radarhere I am fine with your changes to this. Thanks.

[radarhere]

https://github.com/raygard/Pillow/pull/2 keeps the last comment read in info. The thinking is that if GIF comments are not intended to exist on a per-frame basis, then it would seem more correct to keep them when seeking to the the next frame - this way, if there is only a comment at the start of a GIF file, it is present for throughout subsequent seek operations.

With that change, the following code also works before and after this PR, helping backwards compatibility.

from PIL import Image
im = Image.new("RGB", (1, 1))
im2 = Image.new("RGB", (1, 1), "#f00")
im.save("out.gif", save_all=True, append_images=[im2], comment=b"comment")

reloaded = Image.open("out.gif")
reloaded.seek(1)
assert reloaded.info["comment"] == b"comment"