[dvipdfmx] xdvipdfmx/xdv2pdf "x:" specials

[Joseph Wright]

Hello Jonathan (cc dvipdfmx list),

There was a question on TeX-sx yesterday about the "x:" specials used by 
XeTeX/xdvipdfmx 
(https://tex.stackexchange.com/questions/560959/xelatex-graphics-specials). 
I've tried to answer there, tracking stuff through, as today they are 
also available in dvipdfmx. However, I wonder if they are or were ever 
formally documented. I *think* you introduced them in xdv2pdf, but the 
source repo for XeTeX/xdv2pdf only goes back to around 2006, and they 
were already present.

Is there anything you can point to about them?

Regards,

Joseph