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).
http://docs.godotengine.org/en/3.1/tutorials/3d/vertex_animation/animating_thousands_of_fish.html
http://docs.godotengine.org/en/3.1/tutorials/3d/using_multi_mesh_instance.html
The [MultiMesh] resource that will be used and shared among all instances of the [code]MultiMeshInstance[/code].