Fix pushing direction

[Yuma-Mori]

In Ammo's force-pushable example, wherever I click an object, the force comes from (0, 0, 0).

I added vector subtraction to direct force from the clicker to the object. Also, this code works if cursor is mouse.

[diarmidmackenzie]

Thanks for this. I'm happy to merge this as it's clearly an improvement, and this is only test / example code.

A completely correct fix would also allow for the fact that the entity being pushed may not be in world space, so we should use el.object3D.getWorldPostion() rather than el.object3D.position.

It would also be nice to fix up the Cannon version, which looks like it has the same issue.

Further fixes for either of these would be much appreciated!