Create CollisionObject3D debug shapes using RS

[trollodel]

Fixes #48085, fixes #48281

In #45783, I added the possibility to show debug shapes directly in CollisionObject3D. To be conservative (and because it's my first PR), I displayed debug shapes using MeshInstances, as it was done in CollisionShape3D. This unfortunately came with some drawbacks and one of them was described in #48085.

In this PR I replace MeshInstances with direct calls to RenderingServer.
When I test this, the debug shape disappears when the Shape3D resource changes and reappears only when I change the transform. EDIT: fixed
Since I'm not sure if it is my fault or a RS bug, I want to backport it to 3.x soon to see if the same issue appears.

EDIT: I have backported this and the issue doesn't happen, so it's a RS bug.
EDIT 2: See this comment

Bugsquad edit: test scenes here: CollisionObject_debug_meshes.zip

[AndreaCatania]

This seems a good idea, however what happens if the shape size changes? I think this was destroying and creating the mesh each frame for this reason. I think you should listen to the shape changed event, and destroy it if changed.

In alternative, you should consider to change how https://github.com/godotengine/godot/blob/master/scene/resources/shape_3d.cpp#L60 works. So that when the shape changes, if it has a debug mesh, the mesh is updated. (Right now it's just destroyed: https://github.com/godotengine/godot/blob/master/scene/resources/shape_3d.cpp#L98)

[trollodel]

Right now it's just destroyed

I see. I thought it was a RS bug 😅.

[pouleyKetchoupp]

When the shape changes, it gets properly propagated through CollisionShape3D and the owner gets notified, which triggers the re-creation of the debug shape. So with the last changes it seems fine on this aspect.

One thing that's currently not working is updating the transform of the debug shape when the collision object moves. It was handled directly by having a node in the tree before, now it's not (my bad, I completely forgot about that before).

It can be handled in a relatively simple way:

• Add a protected method _on_transform_changed to update the debug shapes transform in CollisionObject3D (it can just return immediately if there's no debug shape).
• It needs to be called on NOTIFICATION_TRANSFORM_CHANGED to handle transform changes from script.
• It also needs to be called in RigidBody3D::_direct_state_changed and PhysicalBone3D::_direct_state_changed to handle transform changes from the physics simulation (the notification is not propagated in this case).

[trollodel]

@AndreaCatania Now I listen for shape changes and the issue I had is gone.

[trollodel]

One thing that's currently not working is updating the transform of the debug shape when the collision object moves. It was handled directly by having a node in the tree before, now it's not (my bad, I completely forgot about that before).

@pouleyKetchoupp Done

[AndreaCatania]

Just some small things to change, but it look nice already. As improvement, if you really feel like, you may consider to add two functions: to add and remove the debug shapes, so you can avoid have a lot of connect / disconnect spread around. But this is not required I think.
Thanks!

[trollodel]

you may consider to add two functions: to add and remove the debug shapes

Debug shapes are added only in one place, so it's useless to add a function for that.

[pouleyKetchoupp]

I've added another comment on a detail, otherwise it all seems to work fine!

[pouleyKetchoupp]

Looks good to me!

Feedback addressed AFAICT, review was otherwise an approval.

[akien-mga]

Thanks!

[akien-mga]

@trollodel Would you be able to make a PR to backport these changes to 3.x? It's not trivially cherry-pickable as is.

[trollodel]

@akien-mga I already have a working backport of this PR (I need to update it after the reviews), I was waiting for this PR to be (finally 😀) merged.