Supporting the .greenshot file format could be a solution. At least for viewing .greenshot files.Editing an annotated image requires creating and editing it in a format that supports that, such as an Adobe image format, which XnView .software doesn't support to the required extent.
The .greenshot file format allows annotations to be edited.
Annotating an image is a great benefit - but annotating with the possibility of editing the annotations would be fantastic.
And Greenshot is open source, so it should be possible to find a way to use it. Could be a win-win situation for XnView & Greenshot.