Fix converted 3.x Skeleton3D missing pose

[nikitalita]

This PR fixes an issue that I missed in my previous PR (#87050), which introduced converting 3.x Skeletons.

For 3.x Skeletons, poses are relative to the rest position. If a pose is set to the rest position value, the pose will be set to default Transform() value. Since this is the default value for pose, this means that the pose property is not saved to the scene.

Thus, when a deprecated Skeleton is loaded, the pose value is not set, and the skeleton position, rotation, and scale will be set to their default values and you'll get funky looking skeletons like this:

Fortunately, there is an easy fix. bound_children is a deprecated property on 3.x skeletons that we weren't doing anything with because it's not present in the current Skeleton3D format (bound children find their parent index automatically in 4.x). However, this property just so happens to be always set on all Skeleton nodes from 3.0 to 3.x branch current regardless of value, and it's always set AFTER both the pose and the rest properties. Thus, when that property is set on the Skeleton3D node, we just check to see if the position, rotation, and scale are their default values, and if so, set the pose to the rest position.

[lyuma]

seems reasonable

[akien-mga]

Thanks!