[NTG-context] Howto have page size to fit with image size

[Patrick Gundlach]

> we have also \startMPpage:
>
> \setupcolors[state=start]
> \starttext
> \startMPpage
>   fill fullcircle scaled 5cm withcolor red ;
> \stopMPpage
> \stoptext


Great! I keep forgetting those.

Patrick