mac/common: add support for transparent rendering

this adds support for possible transparent rendering by setting the
window and layer background to transparent.

NOTE: atm only possible with libplacebo master.

video/out/mac/metal_layer.swift

@@ -84,6 +84,7 @@ class MetalLayer: CAMetalLayer {
             let isForced = newValue == wantsAlpha
             if isOpaque == wantsAlpha || isForced {
                 super.isOpaque = !wantsAlpha
+                backgroundColor = (wantsAlpha ? NSColor.clear : NSColor.black).cgColor
                 log.verbose("Metal layer is opaque (direct-to-display possible): \(isOpaque)" + (isForced ? " (forced)" : ""))
             }
         }

video/out/mac_common.swift

@@ -117,6 +117,10 @@ class MacCommon: Common {
 
     @objc func update(alpha: Bool) {
         layer?.wantsAlpha = alpha
+        DispatchQueue.main.sync {
+            window?.isOpaque = !alpha
+            window?.backgroundColor = alpha ? NSColor.clear : nil
+        }
     }
 
     override func displayLinkCallback(_ displayLink: CVDisplayLink,