Original commit message from CVS:
* ext/libpng/gstpngdec.c: (gst_pngdec_init), (user_info_callback),
(buffer_clip), (user_end_callback), (gst_pngdec_chain),
(gst_pngdec_sink_event), (gst_pngdec_change_state):
* ext/libpng/gstpngdec.h:
Implement buffer clipping/dropping using GstSegment.
This provides accurate seeking.