NOTE: there are two main use-case forFConstraintContainerSolverobjects: the main physics scene and RBAN. The main scene uses the IslandManager to break the scene up into groups of constraints that can be solved in parallel (Islands). Those islands are then put into IslandGroups, and each IslandGroup is solved in a task, therefore we will have oneFConstraintContainerSolverper type of constraint per IslandGroup. RBAN does not attempt to partition its scene into islands and just solves all constraints on its main thread, so it only has oneFConstraintContainerSolver(per constraint type).