baseparse: put buffer in a correct state after gst_adapter_get_buffer call
baseparse: put buffer in a correct state after gst_adapter_get_buffer call

We must make the buffer writable to write its PTS and DTS, and also
reset its duration.

The behaviour is now the same as before commit c3bcbadd, except metas
might still be attached to the buffer extracted from the adapter.

https://bugzilla.gnome.org/show_bug.cgi?id=752092

jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer,krichter722/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer,StreamUtils/gstreamer