Since actors can be active in 3x3 grid around the player, we need to
first load all objects 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 non-animated physics objects.
Animated objects are tied to the scene graph, which doesn't exists yet
in these cells, so we skip them.
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