rsvg-convert adds non-determism to png output

Issue Summary

rsvg-convert adds non-determism to png output

Example SVG

https://github.com/stephenc/tango-icon-theme/blob/master/scalable/apps/internet-news-reader.svg

Librsvg Version

2.56.3

Platform

OpenSUSE Tumbleweed 20230821

Additional logs

cd ~/rpmbuild/BUILD/tango-icon-theme-0.8.90/scalable/apps/ && for i in $(seq 10) ; do
    rsvg-convert -w 128 -h 128 internet-news-reader.svg -o out.png && md5sum out.png
done | sort | uniq -c|wc -l
9

I also ran it without ASLR using setarch -R rsvg-convert and it produced less entropy, but still several different outputs.

This bug was found while working on reproducible builds for openSUSE.