macos: non-native fs keeps track of screen number for change screen comp

Fixes #2370

Comparing NSScreens directly was fragile. It appears that AppKit/Cocoa
can return different instances of NSScreen for the same screen for
unknown reasons between calls to windowDidChangeScreen. I don't fully
understand why this happens.

In any case, our comparison was not safe. Instad, we now keep track of
of the CGDirectDisplayID for each screen and compare those instead.

macos/Sources/Helpers/NSScreen+Extension.swift

@@ -1,6 +1,11 @@
 import Cocoa
 
 extension NSScreen {
+    /// The unique CoreGraphics display ID for this screen.
+    var displayID: UInt32? {
+        deviceDescription[NSDeviceDescriptionKey("NSScreenNumber")] as? UInt32
+    }
+
     // Returns true if the given screen has a visible dock. This isn't
     // point-in-time visible, this is true if the dock is always visible
     // AND present on this screen.