encoder: h264: refine size of coded buffer.
encoder: h264: refine size of coded buffer.

Refine the heuristic to determine the maximum size of a coded buffer
to account for the exact number of slices. set_context_info() is the
last step during codec reconfiguration, no additional change is done
afterwards, so re-using the num_slices field here is fine.

https://bugzilla.gnome.org/show_bug.cgi?id=719953

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