Relax constraints on Y4M header parsing
Relax constraints on Y4M header parsing

Previous parser assumed that the header would not exceed
80 characters. However, with latest FFMPEG changes, the header
of Y4M files can exceed this limit.

New parser can parse an arbitrarily long header, as long each
tag is 255 or less characters.

BUG=aomedia:2876

Change-Id: I9e6e42c50f4e49251dd697eef8036485ad5a1228

webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx