vrawdepay: return output buffer from process
vrawdepay: return output buffer from process

Return the output buffer from the process function instead of pushing
it ourselves. This way, the subclass can actually deal with the return
value of the push.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=693727

Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good