Feature request: add entity:get_original_position()

I need to know the entity's position as it is saved in the map. It's trivial to implement in Lua, but having this function provided would be nice. It's low priority.

My use case is for a Gust Jar type of item. Holding the item button causes objects in its path to shiver left and right by a few pixels. When you release the button it stops. At the end it needs to return to its original position, but if it's too far to the right it'll just stay there.

Example: voadi-vacuum-bug2

I fixed it in Lua by adding entity.original_position right before starting the animation on it, then entity:set_position() upon releasing the button.

How it's actually supposed to work: Screencast_from_2018-09-29_15.14.05