Add AnimationTree Advance Expressions

[fire]

Remake of Reduz's pull request.

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This gives much greater flexibility for creating complex state machines. By directly interfacing with the script code, it is possible to create complex animation advance condition for switching between states.

Ensure assigning AnimationTreeStateMachineTransition base expression node in editor is relative to current AnimationTree node.

Allow setting an expression base node on the AnimationTree itself

Allows specifying an expression as a condition for state machine transitions.

Co-Authored-By: reduz [email protected]

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Part of the V-Sekai group. https://github.com/v-sekai

[fire]

@neikeq This is erroring on mono and I can't figure out why.

[fire]

On master these constants were swapped.

<constant name="PROPERTY_HINT_ARRAY_TYPE" value="40" enum="PropertyHint">
</constant>
<constant name="PROPERTY_HINT_INT_IS_POINTER" value="41" enum="PropertyHint">
</constant>

[reduz]

At a glance it looks ok to me.

[akien-mga]

This is probably a bit tricky at this stage with multiple co-authors not all familiar with all changes, but it would have been nice if the changes in this PR could have been two separate PRs or at least commits, one for the core/expression changes and one actually adding the AnimationTree advance expressions. This would make the code and history more readable and the review of the core and animation parts easier to do for respective teams.
Now it's probably a bit late so I'm not asking for this work to be done a posteriori, unless you're motivated :)

[fire]

Since this has been stuck. I'm motivated!

[fire]

Pulled out Add a const call mode to Object, Variant and Script. #62468

Edited:

Resolved a few comments in the other pr.

The second part is at: https://github.com/V-Sekai/godot/tree/animtree-expressions-separated

[fire]

@TokageItLab Any last reviews?

[TokageItLab]

Sorry, I am not familiar with StateMachine, so I cannot review with confidence.
There is a problem with flickering when first frame in transitions, but I remember that was an issue unrelated to this PR.

[akien-mga]

Thanks!