Implement clone() and dispose() per threejs

[DanielJoyce]

Seems a lot of classes in three.dart are missing clone() and dispose(), making scene copying and manual deallocation of resources basically impossible.

Lets say you attach a texture to a shader material as a uniform, then you want to change the texture later. You can't dispose the current texture, leading to leaks.

Same goes to materials.

You can't clone most things either. So you can't trivially clone a scene and show it on two renderers.