lavfi/separatefields: adjust tb to avoid rounding errors.

Since the PTS need double accuracy, the timebase denominator is doubled
as well, and this new time base is used for the timestamp unit.
x86: sbrdsp: implement SSE2 qmf_pre_shuffle

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

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