TOOLS/lua/ao-null-reload.lua: send ao-reload on audio-device-list change

Only triggers if ao=null. This is required for ao=wasapi to recover from periods
with no available devices.

fixes #2738

TOOLS/lua/ao-null-reload.lua

@@ -0,0 +1,20 @@
+-- Handles the edge case where previous attempts to init audio have failed, but
+-- might start working due to a newly added device. This is required in
+-- particular for ao=wasapi, since the internal IMMNotificationClient code that
+-- normally triggers ao-reload will not be running in this case.
+
+function do_reload()
+    mp.command("ao-reload")
+    reloading = nil
+end
+
+function on_audio_device_list_change()
+    if mp.get_property("current-ao") == "null" and not reloading then
+        mp.msg.verbose("audio-device-list changed: reloading audio")
+        -- avoid calling ao-reload too often
+        reloading = mp.add_timeout(0.5, do_reload)
+    end
+end
+
+mp.set_property("options/audio-fallback-to-null", "yes")
+mp.observe_property("audio-device-list", "native", on_audio_device_list_change)