WebP support

This adds basic support for exporting to the WebP format.  There are still things to be done (XMP/ICC/Exif support) but these will require possibly modification to how DT handles format modules.  Note about the Copyright notice: by contractual obligation, all of my work is copyrighted by my employer (Google Inc.).  A transfer of copyright directly to me is possible but time consuming and I personally do not mind the current status, so please consider accordingly and let me know if that is something you cannot accept.