This commit converts the MatroskaLevel1Element struct to use file-based
offsets, as opposed to the current practice of using offsets relative to
the beginning of the segment in it. This also includes a change from
uint64_t to int64_t.

This is in preparation to another patch that improves the check for
duplicate level 1 elements.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>