Make default background color transparent in wxOSX

This is needed for translucency effects, desktop background images
changing the toplevel window background tint.

Also don't erase background when no background color is specified.

Closes #26301.

include/wx/osx/window.h

@@ -133,6 +133,14 @@ public:
 
 #endif
 
+    virtual wxVisualAttributes GetDefaultAttributes() const override
+    {
+        return GetClassDefaultAttributes(GetWindowVariant());
+    }
+
+    static wxVisualAttributes
+    GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
+
     // implementation from now on
     // --------------------------