ext/libpng/gstpngdec.c: Handle only 8-bppc (bits-per-pixel-component) images, better error handling and correct strid...

Original commit message from CVS:
Reviewed by:  Ronald S. Bultje  <rbultje@ronald.bitfreak.net>
* ext/libpng/gstpngdec.c: (gst_pngdec_src_getcaps),
(gst_pngdec_chain):
Handle only 8-bppc (bits-per-pixel-component) images, better
error handling and correct strides. Fixes #163177.
* ext/libpng/gstpngenc.c: (gst_pngenc_sinklink),
(gst_pngenc_chain):
Better error handling. Fixes #163348.