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 up to ~200 characters. Arbitrary parsing in
future commit.

BUG=aomedia:2876

Change-Id: I2ab8a7930cb5b76004e6731321d0ea20ddf333c1
2 files changed