decoder: h264: skip all pictures prior the first I-frame.
decoder: h264: skip all pictures prior the first I-frame.

Don't try to decode pictures until the first I-frame is received within
the currently active sequence. There is no point is decoding and then
displaying frames with artifacts.

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