Controls the 'granularity' of the Adaptive Binary Tree (ABT) that the
engine uses when a Binary Space Partition (BSP) tree is not available.
The ABT divides the level into rectangular regions; tree_range gives
the average size of a region. The smaller the value, the more 'fine-grained'
is the tree and the less objects are contained in a region. On a dense
populated level, smaller ranges can be advantageous. However a small range
requires frequent tree adaptions when many objects are crossing region
boundaries. So do some experimentation for finding the optimal tree_range for
your project. Note that this value is only a hint for the engine; ABT regions
vary in size depending on the level geometry.
100 .. 10000
var (to be set before loading
tree_range = 2000; // coarse-grained tree, bigger regions