Fix low res macOS icon for dock, app switcher, etc., rendering

Use 1024x1024 size.

cherry-picked from 35edc18

Author: dakcarto

images/icons/qgis-icon-macos.png

@@ -789,7 +789,12 @@ int main( int argc, char *argv[] )
 
   QgsApplication myApp( argc, argv, myUseGuiFlag, configpath );
 
+#ifdef Q_OS_MAC
+  // Set 1024x1024 icon for dock, app switcher, etc., rendering
+  myApp.setWindowIcon( QIcon( QgsApplication::iconsPath() + QLatin1String( "qgis-icon-macos.png" ) ) );
+#else
   myApp.setWindowIcon( QIcon( QgsApplication::appIconPath() ) );
+#endif
 
   //
   // Set up the QSettings environment must be done after qapp is created