avi: Improve non-interleaved detection

Additional fixes by Nigel Touati-Evans <nigel.touatievans@gmail.com>.

Check the index for streams with a time drift of 2s or a buffer drift
of 64MB.

Bug-Id: 666
CC: libav-stable@libav.org
Sample-Id: yet-another-broken-interleaved-avi.avi

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Diego Biurrun <diego@biurrun.de>
