New and improved IK system for Skeleton2D

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.

1 files changed, 52 insertions, 1 deletions

diff --git a/scene/2d/skeleton_2d.h b/scene/2d/skeleton_2d.h
index fd62b87bde..59bd711960 100644
--- a/scene/2d/skeleton_2d.h
+++ b/scene/2d/skeleton_2d.h
@@ -32,6 +32,7 @@
 #define SKELETON_2D_H
 
 #include "scene/2d/node_2d.h"
+#include "scene/resources/skeleton_modification_2d.h"
 
 class Skeleton2D;
 
@@ -46,15 +47,32 @@ class Bone2D : public Node2D {
 	Bone2D *parent_bone = nullptr;
 	Skeleton2D *skeleton = nullptr;
 	Transform2D rest;
-	real_t default_length = 16.0;
+
+	bool autocalculate_length_and_angle = true;
+	float length = 16;
+	float bone_angle = 0;
 
 	int skeleton_index = -1;
 
+	void calculate_length_and_rotation();
+
+#ifdef TOOLS_ENABLED
+	RID editor_gizmo_rid;
+	bool _editor_get_bone_shape(Vector<Vector2> *p_shape, Vector<Vector2> *p_outline_shape, Bone2D *p_other_bone);
+	bool _editor_show_bone_gizmo = true;
+#endif // TOOLS ENABLED
+
 protected:
 	void _notification(int p_what);
 	static void _bind_methods();
+	bool _set(const StringName &p_path, const Variant &p_value);
+	bool _get(const StringName &p_path, Variant &r_ret) const;
+	void _get_property_list(List<PropertyInfo> *p_list) const;
 
 public:
+	Transform2D cache_transform;
+	bool copy_transform_to_cache = true;
+
 	void set_rest(const Transform2D &p_rest);
 	Transform2D get_rest() const;
 	void apply_rest();
@@ -65,11 +83,26 @@ public:
 	void set_default_length(real_t p_length);
 	real_t get_default_length() const;
 
+	void set_autocalculate_length_and_angle(bool p_autocalculate);
+	bool get_autocalculate_length_and_angle() const;
+	void set_length(float p_length);
+	float get_length() const;
+	void set_bone_angle(float p_angle);
+	float get_bone_angle() const;
+
 	int get_index_in_skeleton() const;
 
+#ifdef TOOLS_ENABLED
+	void _editor_set_show_bone_gizmo(bool p_show_gizmo);
+	bool _editor_get_show_bone_gizmo() const;
+#endif // TOOLS_ENABLED
+
 	Bone2D();
+	~Bone2D();
 };
 
+class SkeletonModificationStack2D;
+
 class Skeleton2D : public Node2D {
 	GDCLASS(Skeleton2D, Node2D);
 
@@ -86,6 +119,11 @@ class Skeleton2D : public Node2D {
 		int parent_index = 0;
 		Transform2D accum_transform;
 		Transform2D rest_inverse;
+
+		//Transform2D local_pose_cache;
+		Transform2D local_pose_override;
+		float local_pose_override_amount = 0;
+		bool local_pose_override_persistent = false;
 	};
 
 	Vector<Bone> bones;
@@ -100,15 +138,28 @@ class Skeleton2D : public Node2D {
 
 	RID skeleton;
 
+	Ref<SkeletonModificationStack2D> modification_stack;
+
 protected:
 	void _notification(int p_what);
 	static void _bind_methods();
+	bool _set(const StringName &p_path, const Variant &p_value);
+	bool _get(const StringName &p_path, Variant &r_ret) const;
+	void _get_property_list(List<PropertyInfo> *p_list) const;
 
 public:
 	int get_bone_count() const;
 	Bone2D *get_bone(int p_idx);
 
 	RID get_skeleton() const;
+
+	void set_bone_local_pose_override(int p_bone_idx, Transform2D p_override, float p_amount, bool p_persistent = true);
+	Transform2D get_bone_local_pose_override(int p_bone_idx);
+
+	Ref<SkeletonModificationStack2D> get_modification_stack() const;
+	void set_modification_stack(Ref<SkeletonModificationStack2D> p_stack);
+	void execute_modifications(float p_delta, int p_execution_mode);
+
 	Skeleton2D();
 	~Skeleton2D();
 };