Prevent objects from colliding with own child attachments (#7610)

Also, use a better distance calculation for 'collide with objects'.
Fixes the issue of a vehicle occasionally colliding with its own driver,
causing one of the velocity components to be set to zero.

src/content_cao.h

@@ -139,8 +139,6 @@ public:
 
 	void processInitData(const std::string &data);
 
-	ClientActiveObject *getParent() const;
-
 	bool getCollisionBox(aabb3f *toset) const;
 
 	bool collideWithObjects() const;
@@ -181,6 +179,8 @@ public:
 
 	void setChildrenVisible(bool toset);
 
+	ClientActiveObject *getParent() const;
+
 	void setAttachments();
 
 	void removeFromScene(bool permanent);