webpenc: add support for animated WebP
Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/1715
The libwebp API doesn't match very well with the GstVideoEncoder API, as it only delivers an unframed bitstream once all pictures have been processed, which means we can only push a single buffer manually on our srcpad on finish().
Supporting animated webp is still valuable, and the feature is behind an opt-in property.