Separate multiple GIF comment blocks with newlines

[raygard]

If more than one comment is in a GIF frame, separate them with \r\n in the info dict.

Fixes #6293 .

Changes proposed in this pull request:

• When reading GIF file, multiple comments in a frame are separated with \r\n, rather than being concatenated.
• Add a test to check this for the first frame of an image.
• Add an image to the test set that contains two comment blocks before the first frame (as well as several other comment blocks.)

[radarhere]

If I suggest \n instead of \r\n, how do you feel about that?

[raygard]

As mentioned in the corresponding issue, I found all the cases of more than one comment per GIF that I could find easily in my files. I see one where a single \n is used, but several that use \r\n to separate lines within a comment. I'm fine with any way you want to go on this issue. It's far less important to me than PR #6300, the one that fixes the comment-per-frame problem, that will (along with PR #6211) probably fix the problem @smccombie has with animated GIFs in MS Teams.