ts_offset was added to cluster timecode, but then effectively subtracted
back off the block timecode

When setting initial_padding for an audio stream, the timestamps are
written incorrectly to the mkv file.  cluster timecode gets written
as pts0 + ts_offset which is correct, but then block timecode gets
written as pts - cluster timecode which expanded is
pts - (pts0 + ts_offset).  Adding cluster and block tc back together:
cluster + block = (pts0 + ts_offset) + (pts - (pts0 + ts_offset)) = pts
But the result should be pts + ts_offset since demux will subtract the
CodecDelay element from pts and set initial_padding to CodecDelay.
This patch gives the correct result.