Since https://github.com/odoo/odoo/commit/a473453b3166a381130d9bd6ec4851b9e1ecab26, a newly cropped image gets a base64 src until the user choose to save the page (where we create an attachment for the cropped picture). The problem is that when reentering the crop edition at once... it was not able to be performed because we did not handle base64 images there. This commit solves this and accidentally adds a feature: allow cropping base64 images.