add (Google) webp image support

[BrianInglis]

Please consider adding webp image support from Google.
This format can be considerably smaller than legacy formats greatly benefiting mobile and low bandwidth users.
It would be advantageous to be able to convert images to webp format files and use them in our standard processes where there is a benefit.
We currently convert the images offline using various webp conversion utilities and options and pick the smallest converted files, which must also be smaller than the original image format files, and update the (mainly xml) web page source image references.
We have started migrating Cygwin and newlib web pages to use webp images, but htmldoc is part of some site doc (mainly PDF) generation processes, so we can not easily migrate those pages, without adding complexity including auxiliary webp image conversion, and generated web page href suffix editing processes, after PDF conversion, for those images which benefit from webp conversion.

[michaelrsweet]

@BrianInglis I don't have a strong desire to add this format and would require yet another external library reference to deal with or more custom code for me to maintain in HTMLDOC, which is really in maintenance mode at this point. And honestly if I did add support for webp I'd likely have to just convert it internally to flate-compressed RGB (basically PNG) when embedding in a PDF file, so you'd lose any benefits in the process... :/

I will leave this feature request open, and possibly consider adding webp support to HTMLCSS which is the core of a new generation of HTMLDOC utility, but not something I've had a lot of time to work on lately...