Fix frame-packing==5 with some decoders

The spec mandates that frame-packing==5 requires the SEI on every frame that
begins a view sequence (i.e. the input frames L0-R0-L1-R1 have 4 view sequences,
but if reordered by the encoder to L0-L1-R0-R1 there are now 2 view sequences).
For simplicity, we write the SEI on every frame.

This fixes frame-packing==5 3D playback on some decoders (PlayStation 3, Sony
W8 series, possibly others).
Remove meaningless use of abs()