For node groups there is no good default preview generation.
Nevertheless, t would be useful to generate a preview image for a
node group by rendering an object in some cases.
This commit adds a new operator that allows updating the preview
image for the active asset by rendering the active object.
Note, the operator can also be used for other asset types, not just
node groups.
The operator can be found in a menu right below the refresh-preview
button. Currently it is the only operator in that menu. In the future,
more operators to create previews may be added.
Differential Revision: https://developer.blender.org/D13747
12b26d21b0