Any ideas on how to implement it for png? It doesn't seem trivial using just cairo: https://lists.cairographics.org/archives/cairo/2017-February/027882.html

Ideas

1. Edit file metadata after first writing it to disk using cairo (janky?)
2. Stream output to a variable and then edit that before writing to disk (possible using cairo_surface_write_to_png_stream??)
3. Basically recreate all the cairo png functions here just to add a call to png_set_pHYs

Basically recreate all the cairo png functions here just to add a call to png_set_pHYs

We already do that for other formats, so why not.

Fair enough. I might give it a go then.