mac/view: make our view aware of possible transparent rendering

the default value of isOpaque is false and it was always assumed that
underlaying content needs to be rendered and blended in some way.

make it dependent on the layer's isOpaque property, which theoretically
could increase render performance.

video/out/mac/view.swift

@@ -28,6 +28,14 @@ class View: NSView, CALayerDelegate {
     override var isFlipped: Bool { return true }
     override var acceptsFirstResponder: Bool { return true }
 
+    override var isOpaque: Bool {
+        if let metalLayer = layer as? MetalLayer {
+            return !metalLayer.isOpaque
+        }
+
+        return true
+    }
+
     init(frame: NSRect, common com: Common) {
         common = com
         super.init(frame: frame)