PGML true display math

[Alex-Jordan]

This implements display math for PGML. Note that the current delimiters [:: ::] and [`` ``] make inline math preceded by a \displaystyle, not actual display math. This commit implements a three-colon (ASCIIMath) or three-backtick (LaTeX) version which really does display math, including centering on a new line.

Test with this file:

DOCUMENT();
loadMacros(
  "PGstandard.pl",
  "PGML.pl",
);

BEGIN_PGML
Inline [: sum :]

Displaystyle [:: sum ::]

Display [::: sum :::]

Inline [` \sum `]

Displaystyle [`` \sum ``]

Display [``` \sum ```]

END_PGML

ENDDOCUMENT();

[dpvc]

I'm OK with the changes.

[gajennings]

This seems to work well.