lavf/movenc: make mov_write_stco_tag() not use the offset pos for mode64 heuristic.

At the moment, the moov header is written at the end of the file, so we
can use the current offset (which focus on the end of the mdat already
written) to guess if 64-bits offset will be required or not.

Though, the next commits will make possible the writing of this table at
the beginning, so this heuristic can't work. As a consequence, we check
all the values within the potential offset table for any value >
32-bits.
