Add a setting for 3rd person camera max distance

Currently, camera.lua hardcodes a maximum 3rd person camera distance of 800 game units.

Players may wish to decrease the maximum distance that the third-person camera can zoom out to. For example, some input devices/configurations make it easy to overscroll past someone's desired 3rd person distance, which could be less than 800. Other players may wish to increase the maximum 3rd person distance to more than 800, simply because they want to see more of the world in 3rd person.

Because the maximum is merely defined in camera.lua, which already has a settings page in the scripts menu, it would be easy to "dehardcode" it into a setting.