Perfect PCR compatibility

[lars18th]

Instead of forward the distance from the current PCR and the last PCR over next packets to read, this patch backguards it to the previous packets. So it calculates the needed packets between new PCR packet and the last, and only continues reading packets from this source until the distance is covered. In fact, it slowdowns the reading speed from the input buffer using the PCR marks.

So, now you can use the parameter -m 1 (or -m 3) and achieve the expected behaviour.

This PR supersedes #23, and fixes #18.