mac/vulkan: log opaque state (direct-to-display possible) of metal layer

since we have a mapping layer between vulkan and metal (MoltenVK) we
don't actually know when the layer is opaque and direct-to-display is
possible.

log opaque state.

Author: Akemi

video/out/mac/metal_layer.swift

@@ -76,6 +76,14 @@ class MetalLayer: CAMetalLayer {
         }
     }
 
+    override var isOpaque: Bool {
+       didSet {
+            if isOpaque != oldValue {
+                log.verbose("Metal layer is opaque (direct-to-display possible): \(isOpaque)")
+            }
+        }
+    }
+
     init(common com: MacCommon) {
         common = com
         super.init()