[gimp/gimp-2-10] app: in gimp_drawable_edit_fill(), make trivial alpha-only fill a NOP

[Ell <ell src gnome org>]

commit 5d905d1d653affc4e23d7c362b3b3a72d79002c9
Author: Ell <ell_se yahoo com>
Date:   Mon Dec 31 04:26:25 2018 -0500

    app: in gimp_drawable_edit_fill(), make trivial alpha-only fill a NOP
    
    In gimp_drawable_edit_fill(), if the fill only affects the alpha
    channel, and if the drawable has no alpha channel, or the alpha
    channel is masked out, do nothing, instead of unnecessarily
    performing the fill, which has no effect.
    
    (cherry picked from commit 6384ff01b62f1371759bb40bf68ed648f6f34bdc)

 app/core/gimpdrawable-edit.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)
---
diff --git a/app/core/gimpdrawable-edit.c b/app/core/gimpdrawable-edit.c
index 8b7494d0f4..3456d0bf8a 100644
--- a/app/core/gimpdrawable-edit.c
+++ b/app/core/gimpdrawable-edit.c
@@ -157,7 +157,8 @@ gimp_drawable_edit_fill (GimpDrawable    *drawable,
                          GimpFillOptions *options,
                          const gchar     *undo_desc)
 {
-  gint x, y, width, height;
+  GimpContext *context = GIMP_CONTEXT (options);
+  gint         x, y, width, height;
 
   g_return_if_fail (GIMP_IS_DRAWABLE (drawable));
   g_return_if_fail (gimp_item_is_attached (GIMP_ITEM (drawable)));
@@ -169,6 +170,16 @@ gimp_drawable_edit_fill (GimpDrawable    *drawable,
       return;  /*  nothing to do, but the fill succeeded  */
     }
 
+  if (gimp_layer_mode_is_alpha_only (gimp_context_get_paint_mode (context)))
+    {
+      if (! gimp_drawable_has_alpha (drawable) ||
+          ! (gimp_drawable_get_active_mask (drawable) &
+             GIMP_COMPONENT_MASK_ALPHA))
+        {
+          return;  /*  nothing to do, but the fill succeeded  */
+        }
+    }
+
   if (! undo_desc)
     undo_desc = gimp_fill_options_get_undo_desc (options);
 
@@ -180,12 +191,10 @@ gimp_drawable_edit_fill (GimpDrawable    *drawable,
   else
     {
       GeglBuffer    *buffer;
-      GimpContext   *context;
       gdouble        opacity;
       GimpLayerMode  mode;
       GimpLayerMode  composite_mode;
 
-      context        = GIMP_CONTEXT (options);
       opacity        = gimp_context_get_opacity (context);
       mode           = gimp_context_get_paint_mode (context);
       composite_mode = gimp_layer_mode_get_paint_composite_mode (mode);