ConTeXt template: support font fallback

[lawcho]

Follow up to #9204 (comment).

You write:

---
title: Demo Document
mainfontfallback:
  - EmojiOne Color
  - OpenSymbol
  - XITS Math
---

This empty set comes from OpenSymbol → ∅ ←

But these [stange operators](https://tex.stackexchange.com/a/397674)
aren't in OpenSymbol, so are fetched from XITS Math:

X × ⎰Y ⨿ Z⎱ → ⎰X × Y⎱ ⨿ ⎰X × Z⎱.

Emoji fallback doesn't work 🙁 .

You run:

pandoc src.md -o out.pdf --pdf-engine=context

You get:

Notes:

• You don't need to set mainfont (unlike in the LaTeX template)
• ConTeXt provdes two competing commands for fallback setup:
  • \definefontfallback requires a later call to \definefontsynonym
  • 👉 \definefallbackfamily allows setting fallbacks without knowing the default font
• I couldn't get emoji fallback to work
  • NotoColorEmoji fails to load (with a loca table not foundmtx-context error)
  • EmojiOne Color loads but doesn't substitute anything
• Testing ConTeXt with new fonts is fiddly
  • Must run mtxrun --script fonts --reload
• More tests needed (cc @tarleb)

[tarleb]

Thanks for this. I tried it and didn't succeed in getting emojis working either, and it seems that this is not an easy issue to solve. Apart from that it seems to work really well.

You can avoid the test failures by using a template comment instead of a TeX comment (syntax: $-- comment text). LGTM once that's fixed.

[jgm]

MANUAL.txt should be updated as well.

[lawcho]

You can avoid the test failures by using a template comment instead of a TeX comment (syntax: $-- comment text).

I tried this (in 9384ac3), but the CI still fails.

Probably the comment isn't the root cause, since there are % Ordinary LaTeX comments nearby in the same file.

Could somebody else please fix this, I don't know what I'm doing with pandoc's CI.

[tarleb]

The error comes from the empty line that's added in the output. Removing line 72 should fix the test.