ff_threshold{8,16}_{sse4,avx2} iterate in fixed 16-/32-byte chunks
and over-read/-write past w when w is not a multiple of mmsize.
Split the row at the largest mmsize multiple and run a C tail for
the remainder. Strengthen the checkasm test: sweep widths 1..w-1,
0xAA-sentinel output buffers, multi-row, distinct per-pointer
linesizes, and add depth 10/12 coverage.
Signed-off-by: DROOdotFOO <drew@axol.io>
DROOdotFOO
1d3031535b