* glom/utility_widgets/dialog_image_save_progress.[h|cc]: Replace set_pixbuf()
with set_image_data().
save(): Save the original data, instead of saving the data from the pixbuf.
* glom/utility_widgets/imageglom.cc: Adapt.
This means, for instance, that if the user loads the a JPG in then they will
get exactly the same JPG out.