vc1: handle CLOSED_ENTRY.
vc1: handle CLOSED_ENTRY.

When CLOSED_ENTRY == 0, and if the B pictures that follow an entry-point
lack a reference anchor picture, these B pictures shall be discarded.

https://bugs.freedesktop.org/show_bug.cgi?id=59505

CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi