TeX: offline rendering and image insertion

[adam-p]

See #144 and #261 for background.

We currently (and have always) used a third-party service for TeX rendering -- Google and now CodeCogs. There are two really bad things that come with this:

1. User math is being sent to the third party.
2. If the third party service goes away (like when Google shut down the API we were using), the rendering dies. And not just when writing a new email, but also when reading the email.

The ideal would be to render the math image locally and include it in the rendered MD->HTML output. A big problem with that is that there's no easy way to do it with (for example) Gmail; images with inline data don't work, etc.

However, maybe it's possible to do this by pasting the image; see this comment for details. This is no small task, as it's a fundamental change. It needs a proof of concept that can be tested in multiple email providers and other sites -- there's a very good chance that they don't all support image-pasting and so this method would work worse than the current DOM-insertion-with-remote-image method.

[gtusr]

TeXSend-Gmail userscript works quite well with Markdown Here in Gmail ... so maybe it can be integrated directly into the MHD extension as an additional option.

[adam-p]

I have implemented the paste approach, added local TeX rendering, and I'm hoping for some feedback from people who use TeX. There's a beta extension you can install here (you should disable or uninstall the other extension or else it's going to be confusing). You can find it here:
https://chromewebstore.google.com/detail/markdown-here/enihccblfeliefaickgkbpfeeakjdffh

I'm still cleaning up the code before pushing, but I think it's working well. There are explanatory notes in the options page -- check near the new option and in the changelist.

@cben: Can you try this out? (And everyone else, of course.)

[cben]

Whoa, that's cool! Renders crisp math, works in Gmail, zero communication to CodeCogs — not when composing nor when receiving mail, adjusts size & color to surrounding text, even sets baseline beautifully e.g.:

### below $\frac1{1+\frac1{1+\frac1{1+n}}}$ baseline

> # above $\frac{1+\frac{1+\frac{1+n}1}1}1$ baseline

👏 ✨ 🙏 👏

[cben]

My first feedback is on the config UI.

1. IIUC the way to enable it is enable both checkboxes. If I enable only the experimental one, math is not rendered.
2. IIUC the URL is meaningless in the new mode (despite its checkbox being necessary).
3. The "Privacy Issues:" are not relevant to experimental mode. Same for much of the "customization"?

What do you think of making it radio buttons?

• First a checkbox for enabling math rending at all.
  Under that, 2 radio buttons (all greyed out when off):
  • External rendering (stable).
    [URL text box]
    (Reset to default)
    Privacy issues:
    ...
    Customization and Use:
    ...
  • Clipboard-based rendering (experimental).
    ... same explanations and warning ...
    ("If you don't use TeX math, then there is no reason to enable this feature." can be omitted?)