[poppler] PDF with layers

[Valerio Messina]

HI,
I wanted to point out the lack of support for PDF layers in poppler.
It is a very important function for those who create PDFs for technical 
use. At the moment pdfunite/pdfseparate damage files by merging the 
layer contents on a single layer. To date it seems that only open source 
app is Scribus that is able to produce PDF with layers through its 
internal library, while several programs rely on poppler and are not 
able to produce PDF with layers.

Here are several test files:
https://gitlab.freedesktop.org/poppler/poppler/-/issues/398#note_1305413

thank you in advance,
-- 
Valerio