avfilter/vf_idet: MMX/MMXEXT/SSE2 implementation of idet's filter_line()

integration by Neil Birkbeck, with help from Vitor Sessak.
core SSE2 loop by Skal (pascal.massimino@gmail.com)

Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
