Handle case where timeCode doesn't start at 0

I don't think this is in the Matroska spec, but apparently I may get
files like this.
Write audio with increasing timeCode

I was writing several audio frames with the same timeCode, hoping that
libwebm would put them into blocks of frames. Apparently it won't. So
now I'm writing each audio frame with its own timecode, based on the
samples in it.