* doc/lispref/display.texi (ImageMagick Images): Mention :scale.
(Defining Images): Mention image-scaling-factor.
* lisp/image.el (image-compute-scaling-factor): New function
(bug#22172).
(create-image): Use it.
(image-scaling-factor): New variable.
* src/image.c (compute_image_size): Take :scale into account.