[NTG-context] Set up external figure for particular image type

[Henning Hraban Ramm]

Am 08.07.22 um 17:46 schrieb Thangalin via ntg-context:
> I'd like to change the `maxheight` option for only PNG images, such as:

Define your own figure class with \definexternalfigure:
https://wiki.contextgarden.net/Command/defineexternalfigure

It won’t automatically chose the file format (or maybe it works with 
"order"), but you don’t need to repeat the setting for each instance.

Hraban