Fix animation behaviour when using ReferenceListProperty (#1959)

Fixes issues #1959, when using ReferenceListProperty for the pos in Animations, a tuple
`((initial_position), target_position))` 
is stored in a dictionary with other properties for Animation. This tuple is used to calculate frame-by-frame movements in order to achieve the animation. After each frame calculation, pos (ReferenceListProperty) is updated. The problem was `initial_position` was a reference to pos, and it was modified after each frame.