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Do not load images with gdk-pixbuf; use Rust loaders instead

Federico Mena Quintero requested to merge federico/librsvg:rust-image-loaders-513 into main

This removes the internal use of gdk-pixbuf for decoding images (e.g. when embedded with an element), and uses the image-rs crate instead, which uses Rust-based loaders.

What I hope won't be a breaking change, is that this commit also restricts librsvg to being able to decode only images in these formats:

  • JPEG (required by the SVG spec)
  • PNG (required by the SVG spec)
  • GIF (not required by the spec)
  • WebP (not required by the spec)

This is per the SVG spec's minimum requirements - https://www.w3.org/TR/SVG2/conform.html#ConformingSVGViewers

I don't know if anyone is embedding e.g. TIFFs or BMPs in SVGs rendered with librsvg. We'll find out if people complain!

JPEG decoding is not bit-identical to libjpeg-turbo. In the test suite, these three reference images needed updating, since they use JPEGs:

filters-composite-04-f-ref.png filters-conv-02-f-ref.png filters-conv-04-f-ref.png

This is based on a patch by Emile Snyder.

The purpose of decoding raster images in Rust, rather than with the traditional C libraries, is this:

  • Reduce the amount of memory unsafe code from codecs.
  • Make it easier to use librsvg on Windows and MacOS, where setting up gdk-pixbuf is tricky.
  • Encourage the Rust-based codecs to improve, given more users. There's plenty of interesting work to be done here.
  • Remove one more place where gdk-pixbuf is used in the GNOME platform, with the idea of phasing it outg.
Edited by Federico Mena Quintero

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