plug-ins: Use pre-multiplied alpha for Lighting Effect's...

...bump map settings.
Currently, the Lighting Effect's bump map options uses
the RGB values of transparent pixels as part of its map.
As the Bump map filter does not do this, this patch tries
to make it consistent by using associated alpha for the
bump map Babl format to fix this.

plug-ins/lighting/lighting-image.c

@@ -391,9 +391,9 @@ bumpmap_setup (GimpDrawable *bumpmap)
         }
 
       if (gimp_drawable_is_rgb (bumpmap))
-        bump_format = babl_format ("R'G'B' u8");
+        bump_format = babl_format ("R'aG'aB'aA u8");
       else
-        bump_format = babl_format ("Y' u8"); /* FIXME */
+        bump_format = babl_format ("Y'aA u8"); /* FIXME */
     }
 }