Apply darkening only to overlays and once, closes #828.

Author: Meychi

External/Plugins/SourceControl/Managers/OverlayManager.cs

@@ -174,7 +174,7 @@ static public void Init()
 
         private static Image GetSkin()
         {
-            return ProjectWatcher.Skin; //can be changed by external SC-Plugin
+            return PluginBase.MainForm.ImageSetAdjust(ProjectWatcher.Skin); //can be changed by external SC-Plugin
         }
 
         static public void Reset()
@@ -212,7 +212,6 @@ static public void SetOverlay(VCItemStatus status, TreeNode node, TreeView tree)
                     new Rectangle((int)status * curSize, 0, curSize, curSize), GraphicsUnit.Pixel);
             }
             int index = tree.ImageList.Images.Count;
-            composed = (Bitmap)PluginBase.MainForm.ImageSetAdjust((Image)composed);
             tree.ImageList.Images.Add(composed);
             map[node.ImageIndex] = index;
             node.SelectedImageIndex = node.ImageIndex = index;