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, 12 insertions, 6 deletions

diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index 7d57555ce33..8d75b569edb 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -2122,12 +2122,18 @@ to toggle between displaying the file as an image in the Emacs buffer,
 and displaying its underlying text (or raw byte) representation.
 Additionally you can type @kbd{C-c C-x} (@code{image-toggle-hex-display})
 to toggle between displaying the file as an image in the Emacs buffer,
-and displaying it in hex representation.
-Displaying the file as an image works only if Emacs is compiled with
-support for displaying such images.  If the displayed image is wider
-or taller than the frame, the usual point motion keys (@kbd{C-f},
-@kbd{C-p}, and so forth) cause different parts of the image to be
-displayed.  You can press @kbd{n} (@code{image-next-file}) and @kbd{p}
+and displaying it in hex representation.  Displaying the file as an
+image works only if Emacs is compiled with support for displaying
+such images.
+
+If the displayed image is wider or taller than the frame, the usual
+point motion keys (@kbd{C-f}, @kbd{C-p}, and so forth) cause different
+parts of the image to be displayed.  But by default the image is
+resized automatically to fit to the window.  You can configure this by
+using two options @code{image-auto-resize} and
+@code{image-auto-resize-on-window-resize}.
+
+You can press @kbd{n} (@code{image-next-file}) and @kbd{p}
 (@code{image-previous-file}) to visit the next image file and the
 previous image file in the same directory, respectively.