avfilter/af_compand: add Andrew Kelley to the copyright

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: cosmetics and doxy comment from libavfilter/af_compand_fork.c

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: pts init code from libavfilter/af_compand_fork.c

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: error checks from libavfilter/af_compand_fork.c

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: "use local variable for channels" from libavfilter/af_compand_fork.c

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: switch defaults to libavfilter/af_compand_fork.c

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: cosmetics from libavfilter/af_compand_fork.c

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