This is a continuation of #5888 as I realized that most layer modes were
fixed with my commit b3fc24268a (and follow-up f40dc40cbc) 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
GeglOperation alone. E.g. blend_function() for luma lighten
gimp_operation_layer_mode_blend_luma_lighten_only() would call
gegl_operation_get_source_space() which requires the node to exist.
Similarly for the Luma darken only mode. So I keep both the node and
operation around, and when finalizing, I free the node (which in turn
frees the operation).
Ell > if you are reading our commits, I would really appreciate your
review (or fixes) of my code here! :)
75e6f1062e