iop/soften: Drop locals range and hr, and fixup comment

hr is always equal to radius, and range is only used to calculate hr.
iop/soften: Use SSE for inner blur loop
iop/soften: Apply OpenMP to inner blur loops
src/iop/soften: Use SSE in final output stage
iop/soften: Use _mm intrinsics instead of operator overloading for compatibility
Merge remote-tracking branch 'bruceg/soften-speedups'