Add option to freeze auto dewraping

[kugland]

It would be nice to have an option to freeze the shape of auto dewrap (convert it to manual), so that when I change the resolution of the output, the shape is kept. Currently, one has to edit the shape to make it manual.

I normally prepare the pages using 300dpi, and then, when I’m going to create a PDF, export them as 600dpi; when I do this using auto dewrap, some pages that looked good with 300dpi end up deformed.