* lisp/image.el: Support image scaling with mouse in other buffer.

* lisp/image.el (image-increase-size, image-decrease-size):
Add optional arg position.
(image-mouse-increase-size, image-mouse-decrease-size):
Use '(point-marker)' for arg position.
(image--get-image): Use get-char-property from position if non-nil,
and its buffer.
(image--get-imagemagick-and-warn, image--change-size):
Add optional arg position.