Allow reading of growing avi files (currently being written)

Allow reading of growing avi files (currently being written)

This uses the RIFF header stored size to figure out the expected AVI
file size, instead of the actual file. To work fully it requires handling
failed avio_seek() instead of assuming they always succeed.

Some fate file has been cut off and contains half a frame at the end which
previously was not output during demuxing. This frame is now output to
encoder, thus the fate diff update.

Bug-Id: 261
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>