I opened Tests/LibGfx/test-inputs/png/wide-gamut-only.png in
Preview.app and used File->Export as PDF... to convert it to a PDF.
I then ran
mutool clean -d Tests/LibPDF/wide-gamut-only.pdf \
Tests/LibPDF/wide-gamut-only.pdf
to decompress it, edited by hand to remove padding around the image
and shrunk the page's MediaBox to be as big as the image, and ran the
command above again to fix up binary offsets in the xref table.