buffer.

* modules/mux/mpeg/ts.c:
  - rename the caching= option into shaping= since it more adequately
    describes what it does
  - new use-key-frames option, if enabled then shaping= will be a
    maximum value, if the algorithm find a new GOP it will finish with
    the first picture of the next GOP (helps minimizing the dts delay)
  - fixed a bug where the calculated sending time could be later than
    the real DTS time (meaning the decoder wouldn't get the buffer
    in time)