cairo: follow filename convention for PNG output

Multipage PNG output has a "-pageN" suffix.

Fixes #6404 (closed)

added Patchnew label

approved this merge request

No relevant differences at https://lilypond.gitlab.io/-/lilypond/-/jobs/2846981983/artifacts/test-results/index.html.

added Patchreview label and removed Patchnew label

added Patchcountdown label and removed Patchreview label

added Patchpush label and removed Patchcountdown label

added 44 commits

• 80861b2a...84e33685 - 43 commits from branch master
• 61e9241b - cairo: follow filename convention for PNG output

Compare with previous version

Equal diff for commits 80861b2a and 61e9241b, not resetting patch status.

enabled an automatic merge when the pipeline for 61e9241b succeeds

merged

removed Patchpush label

changed milestone to %2.23.12