Interface for fontdimens (+ subdepth) package

[wspr]

On LuaTeX list Ulrike wrote:

As far as I understand the documentation (and the result of some
tests) luatex uses the relevant \fontdimens only when loading the
font to setup some primitive parameters. If you later want to change
the values you have to change the primitives and not the \fontdimens
(as the subdepth package does).

With later example:

\makeatletter 
\AtBeginDocument{%
  \check@mathfonts
  \Umathsubshiftdown\displaystyle=10pt
  \Umathsubsupshiftdown\displaystyle=2.84pt
  \Umathsubsupvgap\displaystyle=1pt
}

It would make sense to provide some sort of high-level interface here.

Secondly, subdepth should then be altered to use these methods if they are provided.