(b) Destination word accumulation (dwa) for specific Sels.
This code is automatically generated. See, for example,
the code in fmorphgen.1.c and fmorphgenlow.1.c.
These files were generated by running the program
prog/fmorphautogen.c. Results can be checked by comparing dwa
and full image rasterops; e.g., prog/fmorphauto_reg.c.