af_asyncts: fix offset calculation

delta is in samples, not bytes. Also the sample format is not guaranteed
to be planar.

CC:libav-stable@libav.org
x86: sbrdsp: implement SSE2 qmf_pre_shuffle

From 253 to 51 cycles on Arrandale and Win64.
44 cycles on SandyBridge.

Signed-off-by: Anton Khirnov <anton@khirnov.net>