it: add test for preferred buffer scale

src/it/test_ifs/test_compositor.rs

@@ -34,6 +34,7 @@ impl TestCompositor {
             tran: self.tran.clone(),
             server,
             destroyed: Cell::new(false),
+            preferred_buffer_scale: Rc::new(Default::default()),
         });
         self.tran.add_obj(surface.clone())?;
         Ok(surface)