Since actors can be active in 3x3 grid around the player, we need to
first load all statics in a 5x5 grid around the player.
Split load and unloading in 2 phases. Add an mInactiveCells set into the
scene, which contains all cells inside the aforementioned 5x5 grid.
These cells contains only heightfields and physics objects of static
class.
f031a191b8