Bug 793734 - White alpha border after upscaling

In gimp_gegl_apply_scale(), use a CLAMP abyss policy for the scale
op, to avoid leaking transparency into the image when scaling
drawables.

Note that this (intentionally) only affects whole-image/layer
scaling, and not scaling done using any of the transform tools.

app/gegl/gimp-gegl-apply-operation.c

@@ -654,12 +654,13 @@ gimp_gegl_apply_scale (GeglBuffer            *src_buffer,
   g_return_if_fail (GEGL_IS_BUFFER (dest_buffer));
 
   node = gegl_node_new_child (NULL,
-                              "operation", "gegl:scale-ratio",
-                              "origin-x",   0.0,
-                              "origin-y",   0.0,
-                              "sampler",    interpolation_type,
-                              "x",          x,
-                              "y",          y,
+                              "operation",   "gegl:scale-ratio",
+                              "origin-x",     0.0,
+                              "origin-y",     0.0,
+                              "sampler",      interpolation_type,
+                              "abyss-policy", GEGL_ABYSS_CLAMP,
+                              "x",            x,
+                              "y",            y,
                               NULL);
 
   gimp_gegl_apply_operation (src_buffer, progress, undo_desc,