Add support for uploading, editing and saving WebP images when supported by the server.
Add 'image/webp' to supported mime types. Correctly identify WebP images and sizes even when PHP doesn't support WebP. Resize uploaded WebP files (when supported) and use for front end markup.
Props markoheijne, blobfolio, Clorith, joemcgill, atjn, desrosj, spacedmonkey, marylauc, mikeschroder, hellofromtonya, flixos90.
Fixes#35725.
git-svn-id: https://develop.svn.wordpress.org/trunk@50810 602fd350-edb4-49c9-b593-d223f7449a82
