png: add a standalone parser

Useful for reading png images from a pipe.
fate: add utility function to test parser, demuxer, and decoder
fate: add a pngparser test
hevc: Refactor decode_nal_sei_frame_packing_arrangement

Directly set the fields when necessary.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
h264: Refactor decode_frame_packing_arrangement

Directly set the fields when necessary.
apidoc: fix warning from stereo3d.h