Original commit message from CVS:
Reviewed by:  Ronald S. Bultje  <rbultje@ronald.bitfreak.net>
* ext/libpng/gstpngdec.c:
* ext/libpng/gstpngenc.c: (gst_pngenc_base_init),
(gst_pngenc_sinklink), (gst_pngenc_init), (gst_pngenc_chain):
* ext/libpng/gstpngenc.h:
Alpha support (encoder; #163161), mime fixage.