mac: set layer content scale on init

usually the content scale updates automatically, though on init it is
possible the wrong scale is used for the initial rendering leading to a
wrongly sized surface.

properly set the initial content scale of the layer.

video/out/mac/common.swift

@@ -116,6 +116,8 @@ class Common: NSObject {
         window.makeMain()
         window.makeKey()
 
+        view.layer?.contentsScale = window.backingScaleFactor
+
         if !minimized {
             window.orderFront(nil)
         }