Fix AnimationTree state machine start()

[rafallus]

Fixes #41475

[rafallus]

Another side effect of this PR is that the state of the State Machine is preserved even when it becomes inactive. Meaning on "restart" the initial state is not played (as the hint in the "Toggle autoplay..." button suggests). I think this behavior is more convenient, at least I wouldn't expect the State Machine to restart each time it becomes active. Please let me know what your thoughts are about this so I can make changes accordingly.

[fire]

@TokageItLab Can you review this?

[TokageItLab]

@fire Since I am not very familiar with StateMachine, I believe that @SaracenOne or @reduz can provide a more appropriate review.

I don't use AnimationStateMachine in my projects because of its difficulty in managing transitions and lack of versatility such as godotengine/godot-proposals#2200 or godotengine/godot-proposals#2523. Also, since the StateMachine code is independent of the AnimationTree, so I've never cared about its performance or usability.

[SaracenOne]

While the bug described in the linked issue still appears to be present, in what I assume is due to internal changes with how 'start' works in AnimationTrees now, this fix no-longer appears to work in the latest master branch (it may still be salvagable for fixing it in 3.x though). I will close this for now, but hope an updated fix can be found for this issue since I believe it severely limits the ability to serialize AnimationTrees for savegames.

[SaracenOne]

Mistake on my behalf, a recent update in the build system caused my to accidentally test this with the wrong binary; I'm re-reviewing it now.

[SaracenOne]

Tested and seems good!

[akien-mga]

Thanks!