Proper support for custom mass properties in 2D/3D physics bodies

Changes:
-Added support for custom inertia and center of mass in 3D
-Added support for custom center of mass in 2D
-Calculated center of mass from shapes in 2D (same as in 3D)
-Fixed mass properties calculation with disabled shapes in 2D/3D
-Removed first_integration which is not used in 2D and doesn't seem to
make a lot of sense (prevents omit_force_integration to work during the
first frame)
-Support for custom inertia on different axes for RigidBody3D

servers/physics_server_3d.h

@@ -405,14 +405,18 @@ public:
 		BODY_PARAM_BOUNCE,
 		BODY_PARAM_FRICTION,
 		BODY_PARAM_MASS, ///< unused for static, always infinite
+		BODY_PARAM_INERTIA,
+		BODY_PARAM_CENTER_OF_MASS,
 		BODY_PARAM_GRAVITY_SCALE,
 		BODY_PARAM_LINEAR_DAMP,
 		BODY_PARAM_ANGULAR_DAMP,
 		BODY_PARAM_MAX,
 	};
 
-	virtual void body_set_param(RID p_body, BodyParameter p_param, real_t p_value) = 0;
-	virtual real_t body_get_param(RID p_body, BodyParameter p_param) const = 0;
+	virtual void body_set_param(RID p_body, BodyParameter p_param, const Variant &p_value) = 0;
+	virtual Variant body_get_param(RID p_body, BodyParameter p_param) const = 0;
+
+	virtual void body_reset_mass_properties(RID p_body) = 0;
 
 	//state
 	enum BodyState {