AVIDump: fix FFV1 encoding

ffmpeg 2.0 changed requirements for the FFV1 encoder and made them more strict,
requiring more fields of the input frame to be initialized. Explicitly setting
pixfmt, width and height solve the EINVAL issues with FFV1 encoding.

Original fix from http://ffmpeg.org/pipermail/libav-user/2013-October/005759.html
Merge pull request #596 from delroth/master

AVIDump: fix FFV1 encoding