Fix SoftBody3D pinned points breaking when reloading scene

[jirisvd]

Fixes #86308.

The issue is that SoftBody3D::_add_pinned_point() uses pinned_point->spatial_attachment->get_global_transform() to calculate the offset of the pinned point, but getting the global transform of the spatial attachment fails when it's not inside the tree. When calling get_tree().reload_current_scene(), the spatial attachment was not inside the tree when _add_pinned_point() was called, causing the bug described in #86308, as the offset calculation failed and the pinned points became displaced as a result.

I found that disabling the deferred call to SoftBody3D::pin_point() implemented in #71824 and instead calling pin_point() immediately in SoftBody3D::_set_property_pinned_points_attachment() got rid of this issue, but that broke the reparenting issue that motivated the deferred call in the first place.

By adding an is_inside_tree() condition and using the deferred call to pin_point() when inside the tree and calling pin_point() directly when outside of the tree, I managed to get it into a state where both the offset calculation issue (#86308) and the reparenting issue (#71785) are prevented.

Physics squad edit:

• Fixes Error during export of the project with Softbody, also errors in exported project.  #82434

[rburing]

Tested and confirmed this fixes the issues.

The direct code path is used when (re)loading/exporting the scene, and the deferred code path is used in the editor e.g. when reparenting.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[akien-mga]

Cherry-picked for 4.3.1.