Implement #65038: IMAGETYPE_WEBP constant missing

Implement #65038: IMAGETYPE_WEBP constant missing

We add WebP support for getimagesize(), getimagesizefromstring)(),
image_type_to_extension() and image_type_to_mime_type(). For now we
care only about the simple lossy WebP format (VP8) and ignore the
lossless (VP8L) and extended (VP8X) formats. We use image/webp as MIME
type as it appears to be pretty common, even though it is not yet
registered with IANA.

Relevant specifications:
 * <https://developers.google.com/speed/webp/docs/riff_container>
 * <https://tools.ietf.org/html/rfc6386>