In snapshot mode pngenc should output exactly one frame and then return FLOW_EOS to upstream. If upstream sends more input frames before shutting down, it should keep returning FLOW_EOS but not output any more encoded frames. After a flushing seek it should output frames again though. Fixes #3069. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5546>