* lisp/image/image-dired-external.el
(image-dired--check-executable-exists): Improve diagnostics.

* lisp/image/image-dired-external.el
(image-dired-thumbnail-set-image-description): Show more detailed
error messages in case of failure.

* lisp/image/image-dired-external.el (image-dired-pngcrush-thumb):
Fix deletion of intermediate file.
(image-dired-cmd-pngcrush-options)
(image-dired-cmd-create-standard-thumbnail-options): Use %u for
file:// URI.
(image-dired--file-URI): New function.
(image-dired-create-thumb-1, image-dired-create-thumb-2)
(image-dired-set-exif-data): Use it to generate correct URI on
MS-Windows.

* src/w32image.c (Fw32image_create_thumbnail): Copy the file names
before mirroring their slashes.

* lisp/image/image-dired-external.el (w32image-create-thumbnail):
Declare.

* lisp/image/image-dired-external.el (image-dired-create-thumb-2):
Optimize PNG thumbnails.
(image-dired-pngnq-thumb, image-dired-pngcrush-thumb)
(image-dired-optipng-thumb): On MS-Windows, invoke the PNG
optimization programs synchronously.
(image-dired-cmd-create-thumbnail-program)
(image-dired-cmd-create-thumbnail-options)
(image-dired-cmd-pngcrush-program)
(image-dired-cmd-optipng-program)
(image-dired-cmd-create-standard-thumbnail-options)
(image-dired-cmd-rotate-original-program)
(image-dired-temp-rotate-image-file)
(image-dired-cmd-write-exif-data-program)
(image-dired-cmd-write-exif-data-options): Doc fixes.
(image-dired-thumb-queue-run): Don't allow more than 30 concurrent
thumbnail-creation jobs on MS-Windows.

* src/w32image.c (get_encoder_clsid, Fw32image_create_thumbnail)
(globals_of_w32image, syms_of_w32image): New functions.
* src/emacs.c (main): Call 'syms_of_w32image' and
'globals_of_w32image'.
* src/w32term.h (syms_of_w32image, globals_of_w32image): Add
prototypes.

* lisp/image/image-dired.el
(image-dired-thumbnail-display-external): Add a fallback for
MS-Windows.
* lisp/image/image-dired-external.el
(image-dired--probe-thumbnail-cmd): New function.
(image-dired--check-executable-exists): Call it to verify that
"convert" is indeed an Imagemagick program.  New argument FUNC
specifies a function that can be used as an alternative to running
EXECUTABLE.
(image-dired-create-thumb-1): Don't call
'image-dired--check-executable-exists' here, ...
(image-dired-thumb-queue-run): ...call it here, with
'w32image-create-thumbnail' as the alternative function.  If on
MS-Windows and no "convert" command, call
'image-dired-create-thumb-2' instead.
(image-dired-create-thumb-2): New function.

* etc/NEWS: Announce the thumbnail support.