Enable r_lightGridPreload by default or expose it in menu

[wiibur]

This took a lot of searching to find, but r_lightGridPreload defaulting to zero was causing massive frametime spikes on my system that far exceeds the recommended specs. Enabling the preload caused a negligible increase to the up-front loading time and made the game enjoyable to play. This setting is covered in the documentation, but it took me a while to find and I almost gave up. Consider enabling the preload by default to avoid this potential stutters, or at the least expose this setting in the menu so users can find it more easily when troubleshooting performance issues. Thanks!