Add tests to check image scaling functionality

This is in preparation to doing further work in this area to avoid
regressions.

* test/data/image/blank-200x100.png: New file for testing
image scaling.

* test/manual/image-size-tests.el: New file.

test/manual/image-size-tests.el

@@ -0,0 +1,64 @@
+;;; image-size-tests.el -- tests for image scaling
+
+;; Copyright (C) 2017 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;; To test: Load the file and eval (image-size-tests).
+;; A non-erroring result is a success.
+
+;;; Code:
+
+(defmacro im-should (form)
+  `(unless ,form
+     (error "%s didn't succeed" ',form)))
+
+(defun im-image (&rest props)
+  (let ((image-scaling-factor 1))
+    (apply
+     #'create-image
+     (expand-file-name "test/data/image/blank-200x100.png" source-directory)
+     'imagemagick nil props)))
+
+(defun im-compare (image width height)
+  (let ((size (image-size image t)))
+    (and (= (car size) width)
+         (= (cdr size) height))))
+
+(defun image-size-tests ()
+  (unless (imagemagick-types)
+    (error "This only makes sense if ImageMagick is installed"))
+  ;; Default sizes.
+  (im-should (im-compare (im-image) 200 100))
+  ;; Changing one dimension changes the other.
+  (im-should (im-compare (im-image :width 100) 100 50))
+  (im-should (im-compare (im-image :height 50) 100 50))
+  ;; The same with :max-width etc.
+  (im-should (im-compare (im-image :max-width 100) 100 50))
+  (im-should (im-compare (im-image :max-height 50) 100 50))
+  ;; :width wins over :max-width etc
+  (im-should (im-compare (im-image :width 300 :max-width 100) 300 150))
+  (im-should (im-compare (im-image :height 200 :max-height 100) 400 200))
+  ;; Specifying both width and height is fine.
+  (im-should (im-compare (im-image :width 300 :height 50) 300 50))
+  ;; A too-large :max-width (etc) has no effect.
+  (im-should (im-compare (im-image :max-width 300) 200 100))
+  (im-should (im-compare (im-image :max-height 300) 200 100))
+  ;; Both max-width/height.
+  (im-should (im-compare (im-image :max-width 100 :max-height 75) 100 50))
+  (im-should (im-compare (im-image :max-width 100 :max-height 25) 50 25)))
+
+;;; image-size-tests.el ends here