scalePILImage scales image in-place

``plone.scale.scale.scalePILImage`` scales an image in-place, thus modifying the passed-in image parameter. It also returns the scaled image.
This should be changed that ``scalePILImage`` scales on a copy and returning the scaled version, leaving the original untouched.

See: https://github.com/plone/plone.scale/pull/36/files#r376985415