Force 3DS to 320x240 by default

I kind of want to avoid hacks like this, but I suppose I can find a
proper way to do this in the future.

src/Config.cpp

@@ -63,6 +63,10 @@ void DefaultConfigData(CONFIGDATA *conf)
 	// Fun fact: The Linux port added this line:
 	// conf->display_mode = 1;
 
+#ifdef _3DS
+	conf->display_mode = 1;
+#endif
+
 	// Reset joystick settings (as these can't simply be set to 0)
 	conf->bJoystick = TRUE;
 	conf->joystick_button[0] = 2;