[NTG-context] non-integer bodyfont size

[Sanjoy Mahajan]

I'm a bit confused about font sizes.  I had a bunch of missing \Delta
symbols in inline math (e.g. $\Delta x$), which I tracked down to

\setupbodyfont[10.5pt]

The non-integral point size works for text but not for math (the \Delta
turned into a backquote).  How to fix that?

type-siz.tex has several sections to which I could add
\definebodyfontenvironment or \definebodyfont lines.  Which sections
would I add to?  Or can I put the additions outside of that file?  And,
most crucially, what are the magic lines :-)

Thanks for suggestions, which I'll wikify.

-Sanjoy