Fix transparency in toolbar buttons when not using comctl32.dll v6.

Old versions of comctl32.dll don't support alpha in the toolbar image list, so
use the masks only for them.

Closes #2609.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75785 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/toolbar.cpp

@@ -663,7 +663,10 @@ void wxToolBar::CreateDisabledImageList()
                                         (
                                             sizeBitmap.x,
                                             sizeBitmap.y,
-                                            bmpDisabled.GetMask() != NULL,
+                                            // Don't use mask if we have alpha
+                                            // (wxImageList will fall back to
+                                            // mask if alpha not supported)
+                                            !bmpDisabled.HasAlpha(),
                                             GetToolsCount()
                                         );
                 break;