Add level of detail handling

[ianmackenzie]

Perhaps something like

Drawable.withLevelsOfDetail
    [ { accuracy = millimeters 1, drawable = ... }
    , { accuracy = millimeters 5, drawable = ... }
    , { accuracy = millimeters 10, drawable = ... }
    ]

and then the correct drawable would be chosen based on the current camera position. Note that the near clip distance of the camera can be used as a lower bound for how close the object can be to the viewer, to help determine required accuracy in world units based on desired accuracy in pixels!