vaapidecode: use more standard helpers.
vaapidecode: use more standard helpers.

Use g_clear_object() [glib >= 2.28] and gst_caps_replace() helper functions
in more places.

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