Make AnimationNodeAdd actually additive 3.x

[rafallus]

I took #42302 and did the following:

• Adapt it to 3.x branch
• Make it work with Animation::TYPE_VALUE tracks
• Make it work with Animation::TYPE_BEZIER tracks
• Consider input coming from AnimationNodeStateMachine
• Consider input coming from AnimationNodeBlendSpaceXd
• Make add_directly default to true

Animation::TYPE_TRANSFORM already implemented in original PR. Nothing done for Animation::TYPE_METHOD, Animation::TYPE_AUDIO and Animation::TYPE_ANIMATION tracks

Edit: Could fix #37661

[rafallus]

Using AnimationNodeStateMachine and AnimationNodeBlendSpaceXd as inputs to an AnimationNodeAdd2 needs more testing. I used very simple state machines and blend spaces and it worked, but need to check for more complex systems

[darkconsole]

noticed this didn't make it into 3.5 - is it still going to be relevant for a 3.x milestone?

[Calinou]

noticed this didn't make it into 3.5 - is it still going to be relevant for a 3.x milestone?

Yes, as a 3.6 version is planned.

[akien-mga]

@TokageItLab expressed some concerns over the solution in #42302 here: #37661 (comment)

[Riordan-DC]

A suggestion,
I wrote a different solution based on #42302

My only issue with the solution in #42302 is that if we want to add a subtraction mode down the road (which is commonly used with additive animation to create a delta animation) we would need to add another flag to blend_input. Adding the add_directly flag is good enough to ask process_graph to do something other than blending but ultimately the add_directly flag should become a enum for a blending mode (regular blending by default to preserve functionality).

I understand adding a new node to the animation tree will be its own PR but the change I suggest would make adding any new nodes easier in future.

An image showing the sort of workflow you might use Add2 in; with a new Sub2 node.

Snippet from my solution with existing behavior preserved and add direct and subtract modes implemented.
https://github.com/Riordan-DC/godot/blob/c98a9924842f9866ea1916544d3861f146e6f8ea/scene/animation/animation_tree.cpp#L938-L971

Reference information on additive and delta animations I used. Explains why a subtract node later will be necessary:
http://guillaumeblanc.github.io/ozz-animation/samples/additive/

I admit my solution is not perfect and I have an issue with how I am passing the enum down the animation tree. Without the hacky code any child blend nodes will override the mix mode enum I added and break the solution. I guess it is just kind of messy passing a variable down recursive function calls.
https://github.com/Riordan-DC/godot/blob/c98a9924842f9866ea1916544d3861f146e6f8ea/scene/animation/animation_tree.cpp#L147-L183

I hope this suggestion is relevant and helpful.