Original commit message from CVS:
* ext/libpng/gstpngdec.c: (gst_pngdec_init), (buffer_clip),
(gst_pngdec_caps_create_and_set), (gst_pngdec_task),
(gst_pngdec_chain), (gst_pngdec_sink_event),
(gst_pngdec_libpng_init), (gst_pngdec_change_state),
(gst_pngdec_sink_activate_push):
* ext/libpng/gstpngdec.h:
Use statically allocated segment instead of leaking.
Various cleanups.
Fix flush and seek handling.