Use jpeg_write_marker to write comment

[radarhere]

Another suggestion for python-pillow#6774

I found https://www.freedesktop.org/wiki/Software/libjpeg/

You can write special markers immediately following the datastream header by calling jpeg_write_marker() after jpeg_start_compress() and before the first call to jpeg_write_scanlines(). When you do this, the markers appear after the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before all else. Specify the marker type parameter as "JPEG_COM" for COM or "JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write any marker type, but we don't recommend writing any other kinds of marker.) For example, to write a user comment string pointed to by comment_text:

• jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text));

So this PR switches from using the Python extra variable to calling jpeg_write_marker in C. See what you think

[smason]

very fancy!

would be worth switching over to using that functionality for other parts, e.g. the ICC PROFILE?

[radarhere]

We could. It's just a bit more complex as it involves an array of markers, rather than just one.