This PR and commit adds a new IK system for 2D with the Skeleton2D node
that adds several new IK solvers, a way to control bones in a Skeleton2D
node similar to that in Skeleton3D. It also adds additional changes
and functionality.
This work was sponsored by GSoC 2020 and TwistedTwigleg.
Full list of changes:
* Adds a SkeletonModifier2D resource
* This resource is the base where all IK code is written and executed
* Has a function for clamping angles, since it is so commonly used
* Modifiers are unique when duplicated so it works with instancing
* Adds a SkeletonModifierStack2D resource
* This resource manages a series of SkeletonModification2Ds
* This is what the Skeleton2D directly interfaces with to make IK possible
* Adds SkeletonModifier2D resources for LookAt, CCDIK, FABRIK, Jiggle, and TwoBoneIK
* Each modification is in its own file
* There is also a SkeletonModifier2D resource that acts as a stack for using multiple stacks together
* Adds a PhysicalBone2D node
* Works similar to the PhysicalBone3D node, but uses a RigidBody2D node
* Changes to Skeleton2D listed below:
* Skeleton2D now holds a single SkeletonModificationStack2D for IK
* Skeleton2D now has a local_pose_override, which overrides the Bone2D position similar to how the overrides work in Skeleton3D
* Changes to Bone2D listed below:
* The default_length property has been changed to length. Length is the length of the bone to its child bone node
* New bone_angle property, which is the angle the bone has to its first child bone node
* Bone2D caches its transform when not modified by IK for IK interpolation purposes
* Bone2D draws its own editor gizmo, though this is stated to change in the future
* Changes to CanvasItemEditor listed below:
* Bone2D gizmo drawing code removed
* The 2D IK code is removed. Now Bone2D is the only bone system for 2D
* Transform2D now has a looking_at function for rotating to face a position
* Two new node notifications: NOTIFICATION_EDITOR_PRE_SAVE and NOTIFICATION_EDITOR_POST_SAVE
* These notifications only are called in the editor right before and after saving a scene
* Needed for not saving the IK position when executing IK in the editor
* Documentation for all the changes listed above.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
