Support alt text for images in LaTeX output

[capnrefsmmat]

Since 2021, graphicx supports an alt option containing alt text. Pass fig.alt through unless knitr.include_graphics.alt is FALSE (which can be set if e.g. you're using an older TeX Live whose graphicx does not support this.)

Ongoing LaTeX work to support accessible (tagged) PDFs will eventually use this option to embed alt text in PDFs. See https://www.latex-project.org/publications/indexbytopic/pdf/

See also #1967.

I haven't contributed to knitr before, so not sure of a few things:

1. I didn't see tests that checked \includegraphics output. Are there any tests I should change/add?
2. I added the option because graphicx versions before 2021 will throw an error when they get alt text, so some users may want to turn this off. But I defaulted the option to TRUE, so the alt text is included in LaTeX output by default. Should it default to FALSE instead? The new behavior will be surprising to people with old TeX installs, but on the other hand fig.alt is doing nothing for them right now. At the least I could add a FAQ entry for the error they'd get.

[CLAassistant]

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[yihui]

1. You can either add tests to tests/testit/ (you can create a new script there) or add an example .Rnw document in the repo https://github.com/yihui/knitr-examples.
2. I tend to keep it simple for now by not introducing a global option. If users have to use an old version of LaTeX, they still have the option of not using fig.alt.

Thanks!

[capnrefsmmat]

Okay, I've removed the global option and added a test script.

[yihui]

Thanks!