This is a continuation of #5888 as I realized that most layer modes were
fixed with my commit b3fc2426 (and follow-up f40dc40c) but at least
2 were still crashing GIMP: "Luma Lighten/Darken only" modes.

There were 2 bugs here:

* The first bug was that when gimp_operation_layer_mode_real_process()
  ran, gimp_operation_layer_mode_prepare() had not been run yet.
  prepare() is called before the process() of GeglOperation, but it
  would seem the process() of GimpOperationLayerMode on the other end
  happens before GeglOperation's prepare() is run. I am absolutely
  unsure if this is expected or not and have a hard time figuring out
  all the details of the C/C++ cohabitation.
  As a solution, I am moving out the fish caching (the needed part
  inside gimp_operation_layer_mode_real_process()) in its own function
  so that I can easily call it separately before inspecting the fishes.

* The second issue was that some blend functions needed more than a
  G...