Generally render engines can do subframe mixing themselves, but the purpose of subframe mixing in the simulation output node is to support higher quality motion blur with bakes when there are topology-changing operations after the simulation output node. Linear mixing can fill the gaps while maintaining lower memory usage. All point/instance domain attributes are mixed, but mixing is only supported when the domain size is unchanged or when an `id` attribute gives a mapping between elements. Theoretically it may be possible, but nested instance geometry is not mixed in this commit due to the difficulty of finding matching geometries across arbitrary instance hierarchy changes. Attributes that are completely unchanged are ignored using implicit sharing for better performance. Pull Request: blender/blender#107599