Parcat not dropping prefix_sei NAL units for IDR pictures of non-first parallel segments

[crosewarne]

When using parcat to create a sequence-level bitstream from a collection of segment-level bitstreams, it was found that the prefix_sei NAL unit for the IDR picture of each segment was being retained, whereas for segments other than the first one they should be dropped.

This leads to redundant prefix_sei NAL units in the sequence-level bitstream.

See the attachment for a suggested fix.

[crosewarne]

Patch for parcat