Node that instances a [MultiMesh].
[MultiMeshInstance] 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 trees in a forest or grass strands).
https://docs.godotengine.org/en/3.2/tutorials/3d/vertex_animation/animating_thousands_of_fish.html
https://docs.godotengine.org/en/3.2/tutorials/3d/using_multi_mesh_instance.html
https://docs.godotengine.org/en/3.2/tutorials/optimization/using_multimesh.html
The [MultiMesh] resource that will be used and shared among all instances of the [MultiMeshInstance].