struct SetMaterialCmd

Sets/clears a node's material.

Constraints:

- `node_id` refs a `Node` with the has_material characteristic.

- `material_id` refs a `Material`, or nothing.

- if this command causes the target to have both a `Shape` and a `Material`,

then these must be compatible with each other (see README.md regarding

"Shape/Material Compatibility").

Discussion:

In order to be painted, a node requires both a `Shape` and a `Material`.

Without a material, a node can still participate in hit-testing and clipping.

Without a shape, a node cannot do any of the above.