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
x86: videodsp: Properly mark sse2 instructions in emulated_edge_mc as such.

Should fix crashes or corrupt output on pre-SSE2 CPUs when they were
using SSE2-code (e.g. AMD Athlon XP 2400+ or Intel Pentium III) in
hfix or hvar single-edge (left/right) extension functions.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
x86: videodsp: Fix a bug in a %if statement where we used '%%' instead of '&&'.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
atrac3plus: Make initialization dependant on channel count rather than channel map

Makes it easier to recreate an AVCodecContext for ATRAC3+ decoding,
which is needed in multimedia frameworks, as well as in general cases
where demuxing and decoding are separate entities.
lavf: include 60 fps in guessed standard frame rates

Due to what looks like an off-by-one error 60 * 12 * 1001 / 12 * 1001
is not tested as standard frame rate in avformat_find_stream_info().
xvid: switch to xvid mmx idct as soon as possible

The idct implementation cannot be changed after the quantization matrices
are read since it use a different permutaion.

Bug-Id: videolan/7411
fate: add xvid test for custom matrices

Test sample is made from the sample in Bug-Id: videolan/7411