mpegtsenc: Improve PCR generation and output

This fixes PCR drift due to accumulating TS_PACKET_SIZE*8*90000LL/ts->mux_rate each packet, due to rounding errors when mux_rate does not evenly divide 135360000.
This patch also increases the PCR precision to 27 MHz from 90 kHz and takes the location of the PCR data into account (+11 bytes according to the spec).

Originally committed as revision 25864 to svn://svn.ffmpeg.org/ffmpeg/trunk