Fix WebP image support on MS-Windows

Fix WebP image support on MS-Windows

* src/image.c (init_webp_functions) [WINDOWSNT]: Load Demux
functions from the WebPDemux DLL.  Load internal functions where
the public APIs are inline functions defined in the WebP headers.
(WebPAnimDecoderOptionsInit) [WINDOWSNT]: Define to call
'WebPAnimDecoderOptionsInitInternal'.
(WebPDemux): Define to call 'WebPDemuxInternal'.
(WebPAnimDecoderNew): Define to call
'WebPAnimDecoderNewInternal'.
(syms_of_image) <webpdemux>: New symbol.

* lisp/term/w32-win.el (dynamic-library-alist): Add a member for
'webpdemux'.