Fix a crash when overriding the number of render threads

This needs to be done a bit later, after GTK-related libraries have been initialized, since this invokes the static constructors for PintaCore.

Pinta/Main.cs

@@ -66,17 +66,13 @@ public static int Main (string[] args)
 		root_command.AddArgument (files_arg);
 
 		root_command.SetHandler ((threads, files) => {
-			if (threads > 0)
-				Pinta.Core.PintaCore.System.RenderThreads = threads;
-
-			OpenMainWindow (files);
-
+			OpenMainWindow (threads, files);
 		}, threads_option, files_arg);
 
 		return root_command.Invoke (args);
 	}
 
-	private static void OpenMainWindow (IEnumerable<string> files)
+	private static void OpenMainWindow (int threads, IEnumerable<string> files)
 	{
 		GLib.UnhandledException.SetHandler (OnUnhandledException);
 
@@ -97,6 +93,9 @@ private static void OpenMainWindow (IEnumerable<string> files)
 			RegisterForAppleEvents ();
 		}
 
+		if (threads > 0)
+			PintaCore.System.RenderThreads = threads;
+
 		app.OnActivate += (_, _) => {
 			main_window.Activate ();
 			OpenFilesFromCommandLine (files);