macOS: Unbalanced NSEnable/DisableScreenUpdates

As noted in https://codereview.qt-project.org/#/c/238958/, windowDidResize: does not neccecarily have a matching windowWillResize: (e.g. during zoom it doesn't). The result is that we enable screen updates without a matching disable first, resulting in log messages "SLSReenableUpdate: unbalanced enable/disable update".

The screen update disable was added as a workaround for http://openradar.appspot.com/radar?id=4990815088672768, but since we've disabled threaded GL rendering support on macOS, the workaround isn't needed right now. We should:

• Disable it if threaded rendering is disabled.
• make sure to always pair up the enable/disable.