decoder: h264: fix integration of second field into the DPB.
decoder: h264: fix integration of second field into the DPB.

If the new picture to be added to the DPB is not a first field, then
it shall be the second field of the previous picture that was added
before.

This removes the need for dpb_find_picture() now that we track the
immediately preceding decoded picture, in decode order.

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