Don't alter the buffer contents in image-crop by default

Don't alter the buffer contents in image-crop by default

* lisp/image/image-crop.el (image-crop-buffer-text-function):
Allow modes to alter the textual representation.
(image-crop): Delete the complete image data without assuming it's
all on one line (which isn't the case in image-mode, for instance).
(image-crop--crop-image-update, image-crop--insert-image-data):
Use image-crop-buffer-text-function.
(image-crop--default-buffer-text): New default action -- don't
alter the buffer contents (bug#57793).