Add image-auto-resize defcustoms to image-mode.el

* lisp/image-mode.el (image-auto-resize)
(image-auto-resize-on-window-resize): New defcustoms.
(image-mode-map): Bind "sb" to image-transform-fit-both.
(image-mode): Set image-transform-resize to image-auto-resize initially.
(image-mode--setup-mode): Add hook on image-auto-resize-on-window-resize.
(image-toggle-display-image): Check if image-transform-resize is t.
(image-transform-properties): Check image-transform-resize for nil and t.
(image-transform-fit-both): New command.
(image-transform-reset): Reset image-transform-resize to image-auto-resize.

* doc/emacs/files.texi (Image Mode): Mention image-auto-resize and
image-auto-resize-on-window-resize.

https://lists.gnu.org/archive/html/emacs-devel/2020-04/msg01160.html

1 files changed, 4 insertions, 1 deletions

diff --git a/etc/NEWS b/etc/NEWS
index fe8a8d8775b..1d630a3e91b 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -3523,9 +3523,12 @@ functions.
 *** 'image-mode' now uses this library to automatically rotate images
 according to the orientation in the Exif data, if any.
 
++++
 *** In 'image-mode' the image is resized automatically to fit in window.
 The image will resize upon first display and whenever the window's
-dimensions change.
+dimensions change.  Two user options 'image-auto-resize' and
+'image-auto-resize-on-window-resize' can define resizing parameters or
+disable auto-resizing.
 
 ---
 *** New library image-converter.