[gimp] app: in Luminance mode, replace VLAs with gimp-scratch

[Ell <ell src gnome org>]

commit 70b7316ebc49b4b5e1681b45f7c67d734905e42b
Author: Ell <ell_se yahoo com>
Date:   Sat Dec 1 05:31:37 2018 -0500

    app: in Luminance mode, replace VLAs with gimp-scratch
    
    In the Luminance layer-mode, use the scratch allocator for
    allocating temporary buffers, instead of using VLAs.
    GimpOperationLayerMode already allocates data on the stack,
    calculated as not to overflow the stack on any platform, so having
    any of its descendants also allocate big buffers on the stack is
    risky.

 .../layer-modes/gimpoperationlayermode-blend.c     | 31 +++++++++++++---------
 1 file changed, 19 insertions(+), 12 deletions(-)
---
diff --git a/app/operations/layer-modes/gimpoperationlayermode-blend.c 
b/app/operations/layer-modes/gimpoperationlayermode-blend.c
index 5ab6a15ff9..c3fa09f70c 100644
--- a/app/operations/layer-modes/gimpoperationlayermode-blend.c
+++ b/app/operations/layer-modes/gimpoperationlayermode-blend.c
@@ -32,6 +32,8 @@
 
 #include "../operations-types.h"
 
+#include "core/gimp-scratch.h"
+
 #include "gimpoperationlayermode-blend.h"
 
 
@@ -866,20 +868,23 @@ gimp_operation_layer_mode_blend_luminance (const gfloat *in,
                                            gfloat       *comp,
                                            gint          samples)
 {
-  gfloat layer_Y[samples], *layer_Y_p;
-  gfloat in_Y[samples],    *in_Y_p;
+  gfloat *scratch;
+  gfloat *in_Y;
+  gfloat *layer_Y;
 
-  babl_process (babl_fish ("RGBA float", "Y float"), layer, layer_Y, samples);
-  babl_process (babl_fish ("RGBA float", "Y float"), in,    in_Y,    samples);
+  scratch = gimp_scratch_new (gfloat, 2 * samples);
 
-  layer_Y_p = &layer_Y[0];
-  in_Y_p    = &in_Y[0];
+  in_Y    = scratch;
+  layer_Y = scratch + samples;
+
+  babl_process (babl_fish ("RGBA float", "Y float"), in,    in_Y,    samples);
+  babl_process (babl_fish ("RGBA float", "Y float"), layer, layer_Y, samples);
 
   while (samples--)
     {
       if (layer[ALPHA] != 0.0f && in[ALPHA] != 0.0f)
         {
-          gfloat ratio = safe_div (layer_Y_p[0], in_Y_p[0]);
+          gfloat ratio = safe_div (layer_Y[0], in_Y[0]);
           gint   c;
 
           for (c = 0; c < 3; c ++)
@@ -888,12 +893,14 @@ gimp_operation_layer_mode_blend_luminance (const gfloat *in,
 
       comp[ALPHA] = layer[ALPHA];
 
-      comp      += 4;
-      in        += 4;
-      layer     += 4;
-      in_Y_p    ++;
-      layer_Y_p ++;
+      comp    += 4;
+      in      += 4;
+      layer   += 4;
+      in_Y    ++;
+      layer_Y ++;
     }
+
+  gimp_scratch_free (scratch);
 }
 
 void