The recursion depth was checked for equality with a maximum depth,
allowing leaves with more primitives if a certain depth was reached.
However, a single leaf must always use the same material (including
set_smooth), so if a leaf contained multiple materials it was split
anyway. This meant that in the next recursion step the depth was
larger than the cutoff value and it would go back to recursing until
the number of primitives was small enough, ignoring the recursion
depth for the rest of the process.
In certain edge cases this could lead to a stack overflow.
Even with the check changed from 'equality' to 'larger or equal'
this could still fail in the pathological case where every primitive
has another material. But that can't be helped, and it wouldn't
realistically happen either.
Differential Revsision: https://developer.blender.org/D17188
023277359f