decodebin3 drops data on video streams until a keyframe or header is detected, so for Ogg/VP8 we now need to correctly flag and signal keyframes downstream. The first buffer pushed from each src pad also has the HEADER flag set. Fixes playback of https://github.com/web-platform-tests/wpt/raw/master/media/test.ogv in playbin3. Fixes #1418 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4725>