Log Message:
xv: improve webp support, in a pkgsrc patch by me.

- minimum support for loading animation webp (decode only the first frame)
- handle RGBA images properly (libwebp APIs don't convert RGBA to RGB)
  https://bugs.chromium.org/p/webp/issues/detail?id=616
- plug several resouce leaks
- use snprintf(3) rather than sprintf(3)

Bump PKGREVISION.