model_matrix not updated when using manipulators

[wmalara]

• K3D version: 2.15.2
• Python version: 3.8.13
• Operating System: Ubuntu 20.04 (backend), Windows 10 Pro (frontend)

Description

When manipulating an object in plot "manipulate" mode the object's model matrix doesn't get updated and also "observe" callback is not fired.
I found that it's probably because of this commented out code: https://github.com/K3D-tools/K3D-jupyter/blob/main/js/src/providers/threejs/initializers/Manipulate.js#L18
and could probably simply create a PR reversing this, but I guess there is some broader context of this change I don't know :) (46d508b#diff-6863658dc28ee81ff40444d18191cad7547592119a5f4a302ecee062b6bb428f).

Simple example

import k3d
import k3d.platonic

plot = k3d.plot()
cube = k3d.platonic.Cube().mesh
plot += cube
plot

plot.mode = "manipulate"
plot.manipulate_mode = "rotate"
cube.observe(print)

When manipulating the object the callback is not fired and when checking the model_matrix directly:

cube.model_matrix

it's still an identity matrix:

array([[1., 0., 0., 0.],
       [0., 1., 0., 0.],
       [0., 0., 1., 0.],
       [0., 0., 0., 1.]], dtype=float32)