mac/vulkan: fix frame not displayed when paused

add event handler for when window's occlusion state changes and now
indicates some or all of the window is visible.

handling of the event is needed due to the addition of a visibility
check for rendering that was added in commit 5010c24.

Fixes #16693

video/out/mac_common.swift

@@ -168,4 +168,8 @@ class MacCommon: Common {
         layer?.contentsScale = window?.backingScaleFactor ?? 1
         windowDidResize()
     }
+
+    override func windowDidChangeOcclusionState() {
+        flagEvents(VO_EVENT_EXPOSE)
+    }
 }