for some JPEG files PIL does not return DPI, but returns jfif_density

[dziket]

For some JPEG files - for instance some produced by imagemagick - PIL does not return correct DPI, even though DPI is there, in files' headers. For such files img2pdf does not get their DPI, so it assumes it is 96. It causes that when generated PDF is displayed on a screen, pages' sizes are wrong.

To reproduce the problem, generate PDF from
these two JPEG files. When I do it with command:

$ python3 src/img2pdf.py page-1.jpg page-2.jpg -o output.pdf

it produces a PDF file with two pages and when the PDF is displayed on a screen, the sizes of the pages are different. Which is wrong: the dimensions of the two files measured in pixels are different, but they have different DPI, so their dimensions measured in centimeters should be the same.

I already reported a problem to Pillow, but until they fix it, the change in this pull request fixes the problem.

[dziket]

Hm, probably this change is not needed, because Pillow library is going to fix it:
python-pillow/Pillow#8633