Hi,

I'd like to scale something (actually a symbol drawn in TikZ) to the exact 
high of the char 'M'. I found the scale environment for it but struggle to 
measure the correct font height. I found the "definemeasure/measure/measured" 
commands but they define a dimension and are not for doing measurements on 
rendered text. Is there a simple way to get the height a char as a dimension?

MWE:
```
\starttext
% Is it possible to replace 1cm with a common expression?
This is the letter M. \scale[height=1cm]{a} should have the exact same size as 
M.

This should also be true {\tfc with a bigger M. \scale[height=1cm]{a}}.
\stoptext
```

Best
Gerion