avformat/mov: Bypass av_add_index_entry()

The new mov code uses a temporally non sorted index since 4abfa387b8234736f6e0e541951e3d5eb60eb843
and can thus no longer be filled with av_add_index_entry() which expects the index to be sorted.
Reverting 4abfa387b8234736f6e0e541951e3d5eb60eb843 and this commit would be
a alternative fix as would be various other options.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
