mac/vulkan: log display sync active state of metal layer

since we have a mapping layer between vulkan and metal (MoltenVK) we
don't actually know when display sync is activated

log display sync active state.

Author: Akemi

video/out/mac/metal_layer.swift

@@ -68,6 +68,14 @@ class MetalLayer: CAMetalLayer {
         }
     }
 
+    override var displaySyncEnabled: Bool {
+        didSet {
+            if displaySyncEnabled != oldValue {
+                log.verbose("Metal layer display sync \(displaySyncEnabled ? "active" : "inactive")")
+            }
+        }
+    }
+
     init(common com: MacCommon) {
         common = com
         super.init()