fdo#77454: sw: ww8 import: add heuristic to ignore mangled crop values

Older versions of OOo/LO would mangle negative crop values on round-trip
of WW8, which now (since the drawing-layer does not ignore them) causes
the images to be rendered invisible; probably large factors don't make
sense anyway, so ignore the crop if it's 50x the image size, which
happens to work for the bugdoc.

Change-Id: I9f36d37e3be27234554bc91e80bfe719b1ce86af