ivfdec: support y4m output from raw input

The width and height needed to write the Y4M header can be found by
probing the stream with vpx_codec_peek_stream_info(). This also
has the consequence of supporting multiple codecs from raw files
with automatic detections, should we add additional codecs in the
future.

Change-Id: I7522a8f4c7577b6ed9876d744c59cd86d30c6049