Do not load images with gdk-pixbuf
Per https://www.w3.org/TR/SVG2/conform.html#ConformingSVGViewers, the only image formats conforming SVG implementations need to support are PNG and JPEG.
Currently we load images here using gdk-pixbuf in a very simplistic way. (We could do more, like actually look at GdkPixbuf options for color space, etc.) But in general, we write to the loader from an in-memory buffer with the image contents, and that's it. It should be possible to use Rust loaders exclusively.