Scene: Introduce .dispose()

[Mugen87]

Fixes #15485

[aardgoose]

Do you want to also dispose of the references to objects maintained in WebGLRenderLists, similarly keyed by scene.id and camera.id.

Or should render lists be added to the WebGLRenderStates to avoid duplication of code etc even?

[Mugen87]

Let's just add onSceneDispose() in WebGLRenderLists.

[joshpatt2]

Thanks for doing this!!!

[mrdoob]

Beautiful!

[mrdoob]

Thanks!

[mrdoob]

Does this make #12464 redundant?

[Mugen87]

Unfortunately no. Scene.dispose() is intended to dispose a complete scene object. For instance if an app wants to render a different scene than before, users can call this new method in order to clean up related data cached by WebGLRenderer.

#12464 tries to ensures that disposed materials and geometries are actually removed from memory. Because of how WebGLRenderLists works, this is not always the case. Calling Scene.dispose() would solve this issue but it feels strange to dispose a scene if you are still going to use it. Because of this, I think we should resolve the conflicts in #12464 and merge it.