[metal] Retain references with command buffers (gfx-rs#7842)

Author: andyleiserson

wgpu-hal/src/metal/mod.rs

@@ -309,11 +309,19 @@ struct PrivateDisabilities {
     broken_layered_clear_image: bool,
 }
 
-#[derive(Debug, Default)]
+#[derive(Debug)]
 struct Settings {
     retain_command_buffer_references: bool,
 }
 
+impl Default for Settings {
+    fn default() -> Self {
+        Self {
+            retain_command_buffer_references: true,
+        }
+    }
+}
+
 struct AdapterShared {
     device: Mutex<metal::Device>,
     disabilities: PrivateDisabilities,