rtph264pay: Fix delta-unit/discont handling when injecting SPS/PPS
rtph264pay: Fix delta-unit/discont handling when injecting SPS/PPS

Apply the wanted delta-unit and discont to the first packet; following
packets for this frame are always delta units and not discont.

pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good