plug-ins: fix #5305 Lighting Effects crashes when I use a channel as Bumpmap

When a channel is selected as bumpmap, the bump_format can be different
than it was before. However, we always kept the first bump_format, which
could be RGB, in which case trying to use it with a channel caused a
crash in GIMP.

To fix this we always update the bump_format if bumpmap is enabled.

plug-ins/lighting/lighting-image.c

@@ -383,9 +383,12 @@ image_setup (GimpDrawable *drawable,
 void
 bumpmap_setup (GimpDrawable *bumpmap)
 {
-  if (bumpmap && ! bump_buffer)
+  if (bumpmap)
+    {
+      if (! bump_buffer)
         {
           bump_buffer = gimp_drawable_get_buffer (bumpmap);
+        }
 
       if (gimp_drawable_is_rgb (bumpmap))
         bump_format = babl_format ("R'G'B' u8");