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/clientobject.h

@@ -51,6 +51,7 @@ public:
 	virtual scene::ISceneNode *getSceneNode() { return NULL; }
 	virtual scene::IAnimatedMeshSceneNode *getAnimatedMeshSceneNode() { return NULL; }
 	virtual bool isLocalPlayer() const {return false;}
+	virtual ClientActiveObject *getParent() const { return nullptr; };
 	virtual void setAttachments() {}
 	virtual bool doShowSelectionBox(){return true;}