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

@@ -658,6 +658,7 @@ gimp_gegl_apply_scale (GeglBuffer            *src_buffer,
                               "origin-x",     0.0,
                               "origin-y",     0.0,
                               "sampler",      interpolation_type,
+                              "abyss-policy", GEGL_ABYSS_CLAMP,
                               "x",            x,
                               "y",            y,
                               NULL);