[gimp/gimp-2-10] Issue #5267: Cage transform tool causes artifacts.

[Jehan <jehanp src gnome org>]

commit fcefb409e5d02f56295fddc0f2afec3dac66ac6e
Author: Massimo Valentini <mvalentini335 gmail com>
Date:   Thu Aug 5 16:58:55 2021 +0200

    Issue #5267: Cage transform tool causes artifacts.
    
    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.
    
    (cherry picked from commit 3b2a8f79f0a474eb93f6334777b23514f944b03a)

 app/operations/gimpoperationcagecoefcalc.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)
---
diff --git a/app/operations/gimpoperationcagecoefcalc.c b/app/operations/gimpoperationcagecoefcalc.c
index ba8cd3f5e4..9c78054bbe 100644
--- a/app/operations/gimpoperationcagecoefcalc.c
+++ b/app/operations/gimpoperationcagecoefcalc.c
@@ -212,18 +212,18 @@ gimp_operation_cage_coef_calc_process (GeglOperation       *operation,
   n_cage_vertices   = gimp_cage_config_get_n_points (config);
 
   it = gegl_buffer_iterator_new (output, roi, 0, format,
-                                 GEGL_ACCESS_READWRITE, GEGL_ABYSS_NONE, 1);
+                                 GEGL_ACCESS_WRITE, GEGL_ABYSS_NONE, 1);
 
   while (gegl_buffer_iterator_next (it))
     {
       /* iterate inside the roi */
-      gint  n_pixels = it->length;
-      gint  x = it->items[0].roi.x; /* initial x                   */
-      gint  y = it->items[0].roi.y; /*           and y coordinates */
-      gint  j;
-
-      gfloat      *coef = it->items[0].data;
+      gfloat *coef     = it->items[0].data;
+      gint    n_pixels = it->length;
+      gint    x        = it->items[0].roi.x; /* initial x         */
+      gint    y        = it->items[0].roi.y; /* and y coordinates */
+      gint    j;
 
+      memset (coef, 0, sizeof * coef * n_pixels * 2 * n_cage_vertices);
       while(n_pixels--)
         {
           if (gimp_cage_config_point_inside(config, x, y))