UI: Fix crash at render_display while shutdown on macOS

The display context was sometimes created again by a resize event after
destruction from `OBSBasic::closeEvent`. That resulted in the display
context alive until reaching the destructor of the `OBSQTDisplay` class
and caused a crash in `render_display` on macOS. To avoid this, destroy
the display context at the event `SurfaceAboutToBeDestroyed` and let not
create the display context again.

UI/window-basic-main.cpp

@@ -5154,10 +5154,6 @@ void OBSBasic::closeEvent(QCloseEvent *event)
 
 	delete extraBrowsers;
 
-	ui->preview->DestroyDisplay();
-	if (program)
-		program->DestroyDisplay();
-
 	config_set_string(App()->GlobalConfig(), "BasicWindow", "DockState",
 			  saveState().toBase64().constData());