Add WebP image format support (Bug#51296)

Add WebP image format support (Bug#51296)

* configure.ac (--with-webp): New option.
(HAVE_WEBP): New variable.
(emacs_config_features): Add webp.
* src/image.c (enum webp_keyword_index) [HAVE_WEBP]: New enum.
(webp_format) [HAVE_WEBP]: New variable.
(webp_image_p, init_webp_functions, webp_load) [HAVE_WEBP]: New
functions for WebP support.
(image_types) [HAVE_WEBP]: Define WebP format.
(syms_of_image) <Qwebp> [HAVE_WEBP]: New DEFSYM.  Add image type Qwebp.
* src/Makefile.in (LIBIMAGE): Add WEBP_LIBS.

* lisp/files.el (auto-mode-alist):
* lisp/image-file.el (image-file-name-extensions):
* lisp/image.el (image-type-header-regexps)
(image-type-file-name-regexps, image-type-auto-detectable): Add WebP
support.
* lisp/term/w32-win.el (dynamic-library-alist): Add the libwebp DLL.

* INSTALL:
* admin/CPP-DEFINES:
* doc/lispref/display.texi (Image Formats, Other Image Types):
* nt/INSTALL: Document WebP support.

* test/lisp/image-tests.el (image-find-image)
(image-type-from-file-name): Expand tests.
* test/src/image-tests.el (image-tests--files): Add WebP.
(image-tests-image-size/webp, image-tests-image-mask-p/webp)
(image-tests-image-metadata/webp): New tests.
* test/data/image/black.webp: New file.