I somewhat bisected GEGL commits between 0.4.20 and 0.4.24 and found
that the one that introduced the env var GEGL_OPERATION_INIT_OUTPUT is
the first showing the problem.
Reviewer (Jehan) note: so it would be commit 6e9610e65c on GEGL repo.
This fix makes sense as it means that since this commit, the output
buffer could have random values. It's not a problem for any operation
where we fill every value, but I guess it's not the case for
"gimp:cage-coef-calc" which was likely relying on the old behavior of
being 0-initialized.
3b2a8f79f0