Fix BoneAttachment3D signal connection

[bitsawer]

• Fixes Signal connection issues when opening a project using BoneAttachment3D but it isn't the last scene tab #69614

Previous attempt PR (#75907) was never merged as the issue became a bit harder to repro, but it can still happen in normal editor use, including in repro project #69614 (comment) by switching tabs and reloading the project. The code snippet in #81552 can repro this 100% of the time.

The problem is that bound variable which is used to track if the BoneAttachment3D is attached to a Skeleton3D and connected to its bone_pose_changed signal is set immediately, but the connection is made using a deferred call. This leaves one frame of time where the state can get messed up in various ways.

Looks like the deferred connection trick is a pretty rarely used pattern in Godot code base, after a quick search I could only find 4 similar uses, all in editor specific UI code. It was added in #51368 and replaced a more direct call bind_child_node_to_bone(). There might be a real reason why it creates the connection using a deferred call, but currently it's wrong because if BoneAttachment3D or any of its parent node is reparented or moved in a tree during the same frame the connection tracking (bound variable) will end out of sync with the real connection state.

I tested this with some models+MRP and couldn't notice any regressions, but skeleton related interactions can be pretty complex and subtle. Testing this more for example by importing and re-importing complex models which create BoneAttachment3Ds during import is recommended.

[A-Hugh-Mann]

this appears to remove errors about bone signals when adding/removing BoneAttachment3D to/from the scene tree.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.3.