3D Onion skinning not working

[butkeim]

Godot version

3.3.stable

System information

Windows 10

Issue description

Onion skinning is not working for 3D scene.

I did a bit of investigation on my side and suspect this pull #22225 to be reason of the bug, we can see under animation_player_editor_plugin.cpp that the split of forward_draw_over_viewport() function is partially implemented, only the canvas function is present, the spatial one being missing.

This is only an intuition, I did not do any test/deep research.

pinging @RandomShaper as discussed, as he is the one responsible for the implementation of onion skinning.

Steps to reproduce

This can easily be reproduce, following next steps :

• Create a MeshInstance in a Spatial scene.
• Create an AnimationPlayer.
• Create a track modifying, for example, MeshInstance's position.
• Activate Onion skinning
• Play the current animation, see that onion skinning is not working

Minimal reproduction project

No response