Node that instances a [MultiMesh]. [code]MultiMeshInstance[/code] is a specialized node to instance [GeometryInstance]s based on a [MultiMesh] resource. This is useful to optimize the rendering of a high amount of instances of a given mesh (for example tree in a forest or grass strands). The [MultiMesh] resource that will be used and shared among all instances of the [code]MultiMeshInstance[/code].