decoder: hevc: Fill dependent slice segment headers while parsing
decoder: hevc: Fill dependent slice segment headers while parsing

Copy the data into the dependent slice segment header from the
corresponding independent slice segment header during parsing.

Previously the reference to the "previous" independent header was
held through the parsing phase and then dereferenced during the
decoding phase. This caused all dependent headers to be populated
with the data of the AU's last independent header instead of the
proper corresponding header.

https://bugzilla.gnome.org/show_bug.cgi?id=762352

Changes since v1:
- Reworded commit message

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