[metapost] Fix formatting glitches.

[Andreas Scherer]

Am 10.05.24 um 11:40 schrieb Andreas Scherer:
> prepare [the main] source module of MetaPost for inclusion in the 
> https://ctan.org/pkg/knuth-pdf and https://ctan.org/pkg/knuth-hint 
> packages <s>anytime soon</s>.

Done for TeX Live 2025! (Or rather knuth-{pdf,hint} version 2.3, 
possibly later this year. I think I'll amend a few TOC pages first.)

Note: mp.w can easily lose the '\pdfoutput=1' for non-PDF TeX engines. I 
suggest this change:

@x [0] l.26
\pdfoutput=1 \sanitizecommand\MP{MetaPost}
@y
\ifpdflua \pdfoutput=1 \sanitizecommand\MP{MetaPost} \fi
@z

Plain TeX produces a static mp.dvi, XeTeX a dynamic mp.pdf, and HiTeX a 
dynamic mp.hnt.

Happy hacking!
Andreas