Refactor PDF export

[elegaanz]

The PDF exporter now uses a type state to ensure that it is impossible to access context that is not yet available. It also makes extensive use of the pdf_writer::Chunk::rewrite_into API to have each step being more independent, and thus potentially easier to memoize.

The actual behavior of the crate didn't change a lot, everything is just less entangled and hopefully easier to reason about.

TODO:

• adapt Add parameter to select pages to be exported by CLI #4039 for this branch. I will wait for a first round of reviews to be ready before doing that, so that I can be sure of the design that will be adopted in the end and work from that.
• lots of testing, because quite a lot of things have been moved around, and some parts may not work anymore even if what gets written is not supposed to have changed. The test suite passes, but it mostly catches that there are no duplicate object references, I can't really tell if the PDF is valid or not besides that. I've also run qpdf --check on all files generated by the test runner, and no errors were reported.

[LaurenzV]

I ran the test suite through my script that renders a PDF using different viewers, and I noticed that the following file doesn't seem to render correctly anymore:

// Test the different radial gradient features.
---

#square(
  size: 100pt,
  fill: gradient.radial(..color.map.rainbow, space: color.hsl),
)
---

#grid(
  columns: 2,
  square(
    size: 50pt,
    fill: gradient.radial(..color.map.rainbow, space: color.hsl, center: (0%, 0%)),
  ),
  square(
    size: 50pt,
    fill: gradient.radial(..color.map.rainbow, space: color.hsl, center: (0%, 100%)),
  ),
  square(
    size: 50pt,
    fill: gradient.radial(..color.map.rainbow, space: color.hsl, center: (100%, 0%)),
  ),
  square(
    size: 50pt,
    fill: gradient.radial(..color.map.rainbow, space: color.hsl, center: (100%, 100%)),
  ),
)

---

#square(
  size: 50pt,
  fill: gradient.radial(..color.map.rainbow, space: color.hsl, radius: 10%),
)
#square(
  size: 50pt,
  fill: gradient.radial(..color.map.rainbow, space: color.hsl, radius: 72%),
)

---
#circle(
  radius: 25pt,
  fill: gradient.radial(white, rgb("#8fbc8f"), focal-center: (35%, 35%), focal-radius: 5%),
)
#circle(
  radius: 25pt,
  fill: gradient.radial(white, rgb("#8fbc8f"), focal-center: (75%, 35%), focal-radius: 5%),
)

Before:

After:

Other than that everything else seemed fine, although I admittedly only skimmed it. :)

[elegaanz]

Thanks, I'll see if I can reproduce and fix. And I will definitely do some more in depth testing before merging.