engine_spx2html: Add this

This is a new engine for creating HTML from TeX. SPX is "semantically
paginated XDV", a slight variant on the XDV output created by the XeTeX
engine during TeX processing. It looks like the it will be a misnomer:
we're going to ignore the pagination of the XDV file. But whatever.

We add a dependency on a patched version of "pinot", a crate for
analyzing OpenType files. In order to deal with some math properly, we
need to parse some of the special OpenType math tables.

Also, update dependencies in Cargo.lock.

Author: pkgw