Add check for Object3D.add and Object3D.remove

[speigg]

Adds a fast path when the result of Object3D.add or Object3D.remove should be a no-op.

Sometimes it is convenient to use add and remove methods in a context where they will be called each frame, potentially on the same parent/child objects:

function animate() {
  // ...
  const intersection = raycaster.intersectObject(scene, true)[0]
  if (intersection) intersection.object.add(cursor) // <-- may be added to the same object every frame
}

This change prevents unnecessary work in the add and remove methods when sequentially adding/removing the same object to to/from the same parent, including preventing the "added" and "removed" events from being dispatched repeatedly.

[yomboprime]

For this use case I'd rather use cursor.visible and change its position.

[speigg]

That particular use case is a contrived example.

[mrdoob]

function animate() {
  // ...
  const intersection = raycaster.intersectObject(scene, true)[0]
  if (intersection && cursor.parent !== intersection.object) intersection.object.add(cursor);
}