AnimationTree: Compose nodes as resources.

[stevenctl]

Pain Points:

1. I must resort to manually editing tscn text if: I have a large state machine and I want to move it into a blend tree, sub state machine, or any other form of major reorganization
2. There is no way to re-use sub-trees or sub-nodes across different animation trees.

Proposal:

1. Support ext_resource in tres for AnimationNode (e.g. AnimationNodeBlendTree) resources
2. add the ability to drop existing saved AnimationNodes into an animation tree
3. add the ability to save a node/branch of animation tree into it's own resource file

Example: Referencing state machines

Let's say I have two state machines:

• A "main" state machine that has the normal animations
• An "override" state machine that sometimes changes the character's upper body

[gd_resource type="AnimationNodeBlendTree" load_steps=4 format=3 uid="uid://cau8vk4mhiskf"]

[sub_resource type="AnimationNodeStateMachine" id="AnimationNodeStateMachine_2fm0r"]

[sub_resource type="AnimationNodeStateMachine" id="AnimationNodeStateMachine_6b3na"]

[sub_resource type="AnimationNodeBlend2" id="AnimationNodeBlend2_um0qd"]
filter_enabled = true
filters = ["%GeneralSkeleton:Chest", "%GeneralSkeleton:Head", "%GeneralSkeleton:LeftHand", "%GeneralSkeleton:LeftLowerArm", "%GeneralSkeleton:LeftLowerArm_twist_01", "%GeneralSkeleton:LeftShoulder", "%GeneralSkeleton:LeftUpperArm", "%GeneralSkeleton:LeftUpperArm_twist_01", "%GeneralSkeleton:Neck", "%GeneralSkeleton:RightHand", "%GeneralSkeleton:RightLowerArm", "%GeneralSkeleton:RightLowerArm_twist_01", "%GeneralSkeleton:RightShoulder", "%GeneralSkeleton:RightUpperArm", "%GeneralSkeleton:RightUpperArm_twist_01", "%GeneralSkeleton:Spine"]

[resource]
nodes/MainStateMachine/node = SubResource("AnimationNodeStateMachine_2fm0r")
nodes/UpperBodyStateMachine/node = SubResource("AnimationNodeStateMachine_6b3na")
nodes/override_upper_body/node = SubResource("AnimationNodeBlend2_um0qd")
node_connections = [&"output", 0, &"override_upper_body", &"override_upper_body", 0, &"MainStateMachine", &"override_upper_body", 1, &"UpperBodyStateMachine"]

It should be possible to reference other [gd_resource]/.tres resources to compose animation nodes:

 [gd_resource type="AnimationNodeBlendTree" load_steps=4 format=3 uid="uid://cau8vk4mhiskf"]
 
-[sub_resource type="AnimationNodeStateMachine" id="AnimationNodeStateMachine_2fm0r"]
+[ext_resource type="AnimationNodeStateMachine" uid="uid://cyh552hty00sb" path="res://StandardacterAnimations.tres" id="5_00bht"]
 
-[sub_resource type="AnimationNodeStateMachine" id="AnimationNodeStateMachine_6b3na"]
+[ext_resource type="AnimationNodeStateMachine" uid="uid://dg8gcdb0d623j" path="res://OverrideAnimations.tres" id="1_gmyqf"]
 
 [sub_resource type="AnimationNodeBlend2" id="AnimationNodeBlend2_um0qd"]
 filter_enabled = true
 filters = ["%GeneralSkeleton:Chest", "%GeneralSkeleton:Head", "%GeneralSkeleton:LeftHand", "%GeneralSkeleton:LeftLowerArm", "%GeneralSkeleton:LeftLowerArm_twist_01", "%G
eneralSkeleton:LeftShoulder", "%GeneralSkeleton:LeftUpperArm", "%GeneralSkeleton:LeftUpperArm_twist_01", "%GeneralSkeleton:Neck", "%GeneralSkeleton:RightHand", "%GeneralS
keleton:RightLowerArm", "%GeneralSkeleton:RightLowerArm_twist_01", "%GeneralSkeleton:RightShoulder", "%GeneralSkeleton:RightUpperArm", "%GeneralSkeleton:RightUpperArm_twi
st_01", "%GeneralSkeleton:Spine"]
 
 [resource]
-nodes/MainStateMachine/node = SubResource("AnimationNodeStateMachine_2fm0r")
-nodes/UpperBodyStateMachine/node = SubResource("AnimationNodeStateMachine_6b3na")
+nodes/MainStateMachine/node = ExtResource("5_00bht")
+nodes/UpperBodyStateMachine/node = ExtResource("1_gmyqf")
 nodes/override_upper_body/node = SubResource("AnimationNodeBlend2_um0qd")
 node_connections = [&"output", 0, &"override_upper_body", &"override_upper_body", 0, &"MainStateMachine", &"override_upper_body", 1, &"UpperBodyStateMachine"]
(END)

Of course the editor UI doesn't support this today. If you try to make the manual edit above, you may see errors like:

E 0:00:00:0834   load: res://CharacterAnimations.tres:7 - Parse Error: Unknown tag in file: ext_resource
  <C++ Source>   scene/resources/resource_format_text.cpp:814 @ load()