ImageGlom: Do not transform to PNG output when saving.
* 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.