encoder: h264: fix NAL unit types in packed headers.
encoder: h264: fix NAL unit types in packed headers.

Submit Prefix NAL headers (nal_unit_type = 14) before every packed
slice header (nal_unit_type = 1 or 5) only for the base view. In non
base views, a Coded Slice Extension NAL header (nal_unit_type = 20)
is required, with an appropriate nal_unit_header_mvc_extension() in
the NAL header bytes.

https://bugzilla.gnome.org/show_bug.cgi?id=732083

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