Fix GeometryInstance3D Custom AABB assignment in the editor not working

[Calinou]

This also fixes error spam when changing Custom AABB on a MeshInstance3D that has no Mesh resource assigned yet (which is allowed in the editor). This avoids pitfalls when assigning a custom AABB in a script when loading meshes at runtime.

I've tested this in all rendering methods and it works as expected.

• This closes Error when loading scene with custom AABB on a GeometryInstance3D #86369.

Testing project: test_custom_aabb.zip

Before

An AABB that is too small for the mesh is intentionally used to cull objects earlier than intended, so it makes a difference on screen. Here, the custom AABB is ignored when the scene is loaded due to the bug:

simplescreenrecorder-2024-04-09_19.00.48.mp4

After (this PR)

An AABB that is too small for the mesh is intentionally used to cull objects earlier than intended, so it makes a difference on screen. Here, the custom AABB is being obeyed as expected:

simplescreenrecorder-2024-04-09_19.01.05.mp4

[clayjohn]

I guess the issue here is that the custom_aabb is being loaded before the base (mesh etc.) is set on the object.

It seems this code has been present since instance_set_custom_aabb() was added #12645. Which means this bug has probably been around since custom_aabb was exposed #70114

[akien-mga]

Thanks!