Scene content not displayed when embedded in some widget containers

[benbovy]

To reproduce:

• run the notebook ipygany.ipynb on Binder (Jupyterlab 3.0.5, Ipygany 0.5.0)
• show the ipygany scene (3rd cell) in a HBox -> no output (output cell is resized but nothing is shown)
• show the ipygany scene in a VBox -> it works