Reduces the amount of upfront data required for cluster parsing
thus decreasing latency on seek and startup.

The change in the seek-lavf_mkv FATE test is due to incremental
parsing no longer reading as much data as the old parser and
thus not having that additional data to generate index entries
based on keyframes.  Index entries are added correctly as the
file is parsed.

All FATE tests pass and Chrome has been using this patch for ~6
months without issue.

Currently incremental parsing is not supported for files with
SSA tracks since they require merging packets between clusters.
In this case the code falls back to non-incremental parsing.
Signed-off-by: Aaron Colwell <acolwell@chromium.org>
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>