baseparse: Use new gst_adapter_get_buffer() API instead of gst_adapter_map()
baseparse: Use new gst_adapter_get_buffer() API instead of gst_adapter_map()

This preserves GstMeta properly unless the subclass does special things. It's
enough to make h264parse's stream-format/alignment conversion pass through
metas as needed.

https://bugzilla.gnome.org/show_bug.cgi?id=742385

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