ext/libpng/gstpngdec.*: Handle more than one frame if the content is framed, like with png-in-quicktime (#331917).

Original commit message from CVS:
* ext/libpng/gstpngdec.c: (gst_pngdec_init),
(user_endrow_callback), (user_end_callback),
(gst_pngdec_caps_create_and_set), (gst_pngdec_chain),
(gst_pngdec_sink_setcaps), (gst_pngdec_sink_event),
(gst_pngdec_libpng_clear), (gst_pngdec_change_state):
* ext/libpng/gstpngdec.h:
Handle more than one frame if the content is framed,
like with png-in-quicktime (#331917).