[gimp] app: catch Alt-click exact combination on GimpItemTreeView preview.

[Jehan <jehanp src gnome org>]

commit 14b4c08881fd3aea35b3bae5dce339ca7e308169
Author: Jehan <jehan girinstud io>
Date:   Mon Dec 14 00:37:24 2020 +0100

    app: catch Alt-click exact combination on GimpItemTreeView preview.
    
    When Alt-clicking on an item thumbnail (for instance a layer thumbnail),
    we can trigger an "Alpha to Selection" action on the clicked item. Yet
    the check was not accurate and would work on Alt+any modifier(s)+click.
    Let's catch exactly Alt-click only in order to allow for more actions on
    other combinations.

 app/widgets/gimpitemtreeview.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/app/widgets/gimpitemtreeview.c b/app/widgets/gimpitemtreeview.c
index c855bd37ee..e9793d031a 100644
--- a/app/widgets/gimpitemtreeview.c
+++ b/app/widgets/gimpitemtreeview.c
@@ -1783,12 +1783,13 @@ gimp_item_tree_view_item_pre_clicked (GimpCellRendererViewable *cell,
   GimpContainerTreeView *tree_view = GIMP_CONTAINER_TREE_VIEW (item_view);
   GtkTreePath           *path;
   GtkTreeIter            iter;
-  gboolean               handled = FALSE;
+  GdkModifierType        modifiers = gimp_get_all_modifiers_mask ();
+  gboolean               handled   = FALSE;
 
   path = gtk_tree_path_new_from_string (path_str);
 
   if (gtk_tree_model_get_iter (tree_view->model, &iter, path) &&
-      (state & GDK_MOD1_MASK))
+      (state & modifiers) == GDK_MOD1_MASK)
     {
       GimpImage        *image    = gimp_item_tree_view_get_image (item_view);
       GimpViewRenderer *renderer = NULL;