h264 decoder: Implement reset() for faster flush
h264 decoder: Implement reset() for faster flush

Implement a custom reset() function for faster flushes
that just clear the reference pictures but don't reallocate
the DPB or clear out SPS/PPS

https://bugzilla.gnome.org/show_bug.cgi?id=781142

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi