Add AnimatableBody inherited from StaticBody for moving platforms

Instead of having a physics node named Static that can be either Static
or Kinematic, AnimatableBody is added again as a separate node:
-Inherited from StaticBody to make its usage clearer
-Still separated from CharacterBody to make its usage more focused

Properly implemented constant velocity for kinematic bodies in godot
physics servers (induced velocity without actually moving).

Also updated description for the different physics nodes to make their
usage clearer.

4 files changed, 16 insertions, 8 deletions

diff --git a/servers/physics_2d/body_2d_sw.cpp b/servers/physics_2d/body_2d_sw.cpp
index 64f71fd25c..d86943e658 100644
--- a/servers/physics_2d/body_2d_sw.cpp
+++ b/servers/physics_2d/body_2d_sw.cpp
@@ -269,11 +269,13 @@ void Body2DSW::set_state(PhysicsServer2D::BodyState p_state, const Variant &p_va
 		} break;
 		case PhysicsServer2D::BODY_STATE_LINEAR_VELOCITY: {
 			linear_velocity = p_variant;
+			constant_linear_velocity = linear_velocity;
 			wakeup();
 
 		} break;
 		case PhysicsServer2D::BODY_STATE_ANGULAR_VELOCITY: {
 			angular_velocity = p_variant;
+			constant_angular_velocity = angular_velocity;
 			wakeup();
 
 		} break;
@@ -429,10 +431,10 @@ void Body2DSW::integrate_forces(real_t p_step) {
 	if (mode == PhysicsServer2D::BODY_MODE_KINEMATIC) {
 		//compute motion, angular and etc. velocities from prev transform
 		motion = new_transform.get_origin() - get_transform().get_origin();
-		linear_velocity = motion / p_step;
+		linear_velocity = constant_linear_velocity + motion / p_step;
 
 		real_t rot = new_transform.get_rotation() - get_transform().get_rotation();
-		angular_velocity = remainder(rot, 2.0 * Math_PI) / p_step;
+		angular_velocity = constant_angular_velocity + remainder(rot, 2.0 * Math_PI) / p_step;
 
 		do_motion = true;
 
@@ -621,8 +623,6 @@ Body2DSW::Body2DSW() :
 		direct_state_query_list(this) {
 	mode = PhysicsServer2D::BODY_MODE_DYNAMIC;
 	active = true;
-	angular_velocity = 0;
-	biased_angular_velocity = 0;
 	mass = 1;
 	inertia = 0;
 	user_inertia = false;
diff --git a/servers/physics_2d/body_2d_sw.h b/servers/physics_2d/body_2d_sw.h
index 9cd53ceca1..b83afdbec6 100644
--- a/servers/physics_2d/body_2d_sw.h
+++ b/servers/physics_2d/body_2d_sw.h
@@ -44,10 +44,13 @@ class Body2DSW : public CollisionObject2DSW {
 	PhysicsServer2D::BodyMode mode;
 
 	Vector2 biased_linear_velocity;
-	real_t biased_angular_velocity;
+	real_t biased_angular_velocity = 0.0;
 
 	Vector2 linear_velocity;
-	real_t angular_velocity;
+	real_t angular_velocity = 0.0;
+
+	Vector2 constant_linear_velocity;
+	real_t constant_angular_velocity = 0.0;
 
 	real_t linear_damp;
 	real_t angular_damp;
diff --git a/servers/physics_3d/body_3d_sw.cpp b/servers/physics_3d/body_3d_sw.cpp
index 7f62e4eb85..6640245d91 100644
--- a/servers/physics_3d/body_3d_sw.cpp
+++ b/servers/physics_3d/body_3d_sw.cpp
@@ -302,10 +302,12 @@ void Body3DSW::set_state(PhysicsServer3D::BodyState p_state, const Variant &p_va
 		} break;
 		case PhysicsServer3D::BODY_STATE_LINEAR_VELOCITY: {
 			linear_velocity = p_variant;
+			constant_linear_velocity = linear_velocity;
 			wakeup();
 		} break;
 		case PhysicsServer3D::BODY_STATE_ANGULAR_VELOCITY: {
 			angular_velocity = p_variant;
+			constant_angular_velocity = angular_velocity;
 			wakeup();
 
 		} break;
@@ -473,7 +475,7 @@ void Body3DSW::integrate_forces(real_t p_step) {
 		//compute motion, angular and etc. velocities from prev transform
 		motion = new_transform.origin - get_transform().origin;
 		do_motion = true;
-		linear_velocity = motion / p_step;
+		linear_velocity = constant_linear_velocity + motion / p_step;
 
 		//compute a FAKE angular velocity, not so easy
 		Basis rot = new_transform.basis.orthonormalized() * get_transform().basis.orthonormalized().transposed();
@@ -482,7 +484,7 @@ void Body3DSW::integrate_forces(real_t p_step) {
 
 		rot.get_axis_angle(axis, angle);
 		axis.normalize();
-		angular_velocity = axis * (angle / p_step);
+		angular_velocity = constant_angular_velocity + axis * (angle / p_step);
 	} else {
 		if (!omit_force_integration && !first_integration) {
 			//overridden by direct state query
diff --git a/servers/physics_3d/body_3d_sw.h b/servers/physics_3d/body_3d_sw.h
index f58f40652b..38b7c4f94c 100644
--- a/servers/physics_3d/body_3d_sw.h
+++ b/servers/physics_3d/body_3d_sw.h
@@ -44,6 +44,9 @@ class Body3DSW : public CollisionObject3DSW {
 	Vector3 linear_velocity;
 	Vector3 angular_velocity;
 
+	Vector3 constant_linear_velocity;
+	Vector3 constant_angular_velocity;
+
 	Vector3 biased_linear_velocity;
 	Vector3 biased_angular_velocity;
 	real_t mass;