Quality issues introduced by qpdf flattening in pdftopdf filter and another issue with switching to ghostscript due to failed hasAcroForm check

[kabassanov]

Describe the bug
Hi,
I got some users complaining that documents were printed in bad quality (in particular they were paler than others). After investigations, I found these documents arrived in pdf format, then were transformed by pdftopdf and pdftops, and have been finaly printed. It appeared that this quality deterioration was related to pdftopdf and its default qpdf processing. Documents were normaly defined for 600dpi printing. So I tried to switch processor from QPDF to Ghostscript, but initially nothing changed. After some debugging, I remarked that when qpdf_flatten is not enabled (gs_flatten is enabled), there is an additional check "hasAcroForm" and that it fails. So I wonder if/why this test is needed... As far as I tested till now, it seems that disabling this additional check allows to switch to ghostscript and to solve this quality issue...

To Reproduce
Steps to reproduce the behavior:

1. Try printing a page from a linux (driverless) or MacOS (Neighborhood printer) client
   (example https://www.bealecorner.org/red/test-patterns/printer-respat.pdf) through a remote cups server
2. Issue lpadmin -p TEST-PRINTER -o pdftopdf-form-flattening-default=ghostscript on the server
3. Try printing again
4. Comment this additional check
   pdftopdf.cc.patch on the server
5. Try printing again
6. Compare printed pages

Expected behavior
Get reasonable quality for printed documents (asking for some precision)...

System Information:

• OS: Server Ubuntu avec cups
• Version 22.04 LTS