Use instanced rendering for the building blocks. Disabled the building placeholders. Detect some prefab types which can be grouped automatically, mostly just mesh colliders used by the building components. Added a GibbableAttribute so that building components don't have to duplicate that information in every single instance (this also improve pool reuse). Made Gibbables use a gib.prefab and made them poolable. Removed ConstructionSkin component and moved the logic into BuildingBlock.