多个片段合并成一个：
ffmpeg -i a.flv \
-vf "select='between(t,0,34)+between(t,170,177)+between(t,400,465)+between(t,475,603)',
setpts=N/FRAME_RATE/TB" \
-af "aselect='between(t,0,34)+between(t,170,177)+between(t,400,465)+between(t,475,603)',
asetpts=N/SR/TB" out.flv