Fast, async, upgraded LaTeX previews for Org-mode

Note: This library is a work in progress. Expect glitches and breakages!

Load, run M-x org-preview-mode. Use Org as usual. Be amazed.

org-preview-2.mp4

Lots to do! [3/18]

[X] Basic equation numbering in previews
[X] Basic color matching (Match default foreground/background of the theme)
[X] Async support for dvisvgm. ~~(Only supports =dvipng= right now.)~~
[-] Code organization, refactor into task-specific functions?
[ ] Add back image caching. (Disabled by design right now.)
[ ] Async method (process sentinels vs org-async vs custom macro)
[ ] Equation ↔ Image mismatch handling
[ ] Overlay placement mismatch handling (async problem)
[ ] Scale matching
[ ] Advanced color matching (Match colors at point)
[ ] Formula ↔ Text baseline alignment (for png, svg)
[ ] Preamble precompilation
[ ] Automatic preview toggle on cursor movement
[ ] Async support for imagemagick.
[ ] Split preview code from org.el (Modularize Org)
[ ] [Advanced] Consistent equation numbering by section in previews.
[ ] Try one TeX run + multiple image-converter processes (more overhead, but threading!)
[ ] Fix Org’s code verbosity with macros from Emacs 26+ (when-let, pcase-let, etc)