lavf: remove unreliable timestamp guessing heuristic
lavc: don't set AVFrame.pts to random numbers in decoders.
ac3dec: fix outptr increment.

Fixes corrupt data errors when downmixing in the AC-3 decoder.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

CC:libav-stable@libav.org
lavc: remove old unused audio conversion functions.
lavf: do not export av_register_{rtp,rdt}_dynamic_payload_handlers from shared objects
lavfi: do not export the filters from shared objects