movenc: Place the sidx index after the initial moov/mdat pair

movenc: Place the sidx index after the initial moov/mdat pair

For fragmented files with non-empty moov, with a fragment index
(sidx), place the index after the initial moov/mdat pair.

Previously, for this pathological case, the index was written
at the start of the file.

Signed-off-by: Martin Storsjö <martin@martin.st>