rtpvp9depay: Improve SVC parsing, aggregate all layers
- Fix start and end of picture to support multiple layers. Start of
picture is the first packet of the base layer, while end of picture
is when the marker bit is set (last packet of the enhancement
layers).
- All "layers" (aka "frames") of a picture are pushed downstream in a
single buffer when picture is complete.
- Forgive SID=0 for enhancement layers (invalid, but Chrome and
Firefox sends it)
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/773>