ac3dec: Allow asymmetric application of DRC when drc_scale > 1

Amplification of quiet sounds is enhanced.
Inspired by gbooker's A52Decoder.
doc: add decoders.texi

Only documents ac3 decoder options at this point.
doc: document correct option to list encoders
Merge commit '23d461fe8714a20ee5e6929f22c61512fdda568e'

* commit '23d461fe8714a20ee5e6929f22c61512fdda568e':
  ac3dec: Allow asymmetric application of DRC when drc_scale > 1

Conflicts:
	libavcodec/ac3dec.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'a17ab0e46a9fec7c31cc1be363828184c6ecebf7'

* commit 'a17ab0e46a9fec7c31cc1be363828184c6ecebf7':
  doc: add decoders.texi

Conflicts:
	doc/decoders.texi
	doc/ffmpeg.texi
	doc/ffplay.texi

See: 5e45f03420945dfeaf66729f5d022e5c15ff5723
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  doc: document correct option to list encoders

Conflicts:
	doc/encoders.texi

Merged-by: Michael Niedermayer <michaelni@gmx.at>
x86/vp9lpf: add ff_vp9_loop_filter_h_{48,84}_16_{sse2,ssse3,avx}().

5.40s → 5.30s overall decode time with -threads 1 on ped1080p.webm
(i7 920, ssse3)
ffmpeg: also count data streams bytes

Fixes wrong error message about empty output file

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>