PDB function gimp_edit_blend() was based on "gimp:shapeburst" operation
whereas the rest of GIMP (in particular, the Blend tool) used
"gegl:distance-transform" which is much faster.
Setting the operation to "manhattan" metric ensures that it still
renders the same way as in 2.8 while being a lot faster.

There was still a problem regarding as how it renders differently from
the Blend tool, but it turns out that the Blend tool is the one
rendering differently from how it used to in 2.8. We should discuss
adding the "metric" property in the tool options.