fix playback sound on Windows after SDL upgrade

SDL in the Windows build was upgraded from version 2.0.5 to version
2.28.1 to fix a bug where sound is not follow changes to the default
output device in Windows

https://forum.shotcut.org/t/sound-into-headphones/33298
https://forum.shotcut.org/t/help-shotcut-wont-switch-audio-playback-
devices/22038
https://forum.shotcut.org/t/audio-output-does-not-change-with-computer-
on-windows/14549
https://forum.shotcut.org/t/shotcut-will-not-play-audio-through-
bluetooth-headphones/11112

SDL version 2.0.6 fixes this but also introduced the default driver
"wasapi" that creates a buzzing or crackling sound when the system is
configured for 5.1. It also plays stereo as multi-channel stereo like
many surround receivers offer but even when the receiver is not
configured as such! "directsound" fixes those issues, but a different
kind of distortion appears after changing output devices. "winmm" fixes
that, but it does not work when opened with more than 2 channels. What a
mess.

src/mainwindow.cpp

@@ -1577,6 +1577,11 @@ bool MainWindow::open(QString url, const Mlt::Properties *properties, bool play)
             mlt_properties_inherit(MLT.producer()->get_properties(), props->get_properties());
         m_player->setPauseAfterOpen(!play || !MLT.isClip());
 
+#if defined(Q_OS_WIN)
+        if (!::qEnvironmentVariableIsSet("SDL_AUDIODRIVER")) {
+            ::qputenv("SDL_AUDIODRIVER", MLT.audioChannels() > 2 ? "directsound" : "winmm");
+        }
+#endif
         setAudioChannels(MLT.audioChannels());
         if (url.endsWith(".mlt") || url.endsWith(".xml")) {
             setVideoModeMenu();
@@ -3206,7 +3211,7 @@ void MainWindow::on_actionProgressive_triggered(bool checked)
 
 void MainWindow::changeAudioChannels(bool checked, int channels)
 {
-    if ( checked ) {
+    if (checked) {
         Settings.setPlayerAudioChannels(channels);
         setAudioChannels(Settings.playerAudioChannels());
     }