Unbounded memory usage when an object falls indefinitely

[sebcrozet]

If an object falls indefinitely, the memory usage of Rapier will increase steadily. This is likely caused by the broad-phase allocating empty grid cells traversed by the falling object.

We should see if there is a way to automatically free the empty broad-phase grid cells without any performance impact.

[stephanemagnenat]

As a user, I would say that whether one wants to release empty cells depends on the application: if the world is bounded and small, one wants to keep empty cells to avoid repeating de-allocation and re-allocation. If the world is unbounded or large, one wants to free empty cells, possibly after a give unused time. Of course, if the de-allocation is very cheap, de-allocating cells every time a contained object counter reaches 0 seems acceptable. If it is not that cheap, maybe it is best to leave the decision to the user, either through a global flag enabling automatic freeing of empty cells, or through an explicit cell garbage collection call.

[sebcrozet]

Letting the user decide sounds like a good idea. We can either add a parameter to the broad-phase constructor, or enable the de-allocation by default (to avoid any surprises memory-wise) and add a method to let the user disable it explicitly.