Investigate if it's possible to embed scale in image metadata

[emersion]

• http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.8
• Ability to embed dpi metadata in images mapnik/mapnik#2322

[ammgws]

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

[emersion]

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.

[ammgws]

Fair enough. I might give it a go then.