Fix: StructuralSparseSet.Clear

[mikhail-dcl]

StructuralSparseArray was improperly cleared as it used Size that refers to the actual number of entities instead of the length of the Entities array itself.

In CommandBuffer, it was leading to leftovers from the previous iteration if it was re-used after Playback resulting in the inconsistent state.

[genaray]

Thanks! Merged #273 which fixes the same issue but also adds a unit test :)