Unexpected possibility to drop elements to unexpected regions when a draggable element is placed another draggable element

[sunpietro]

Unexpected possibility to drop elements to unexpected regions when a draggable element is placed another draggable element.

Steps to reproduce the issue:

1. Create a new instance of Dragster.js that limits regions and draggable elements,
2. Inside any of draggable elements implement interface that allows to drag'n'drop elements inside,
3. Create another instance of Dragster.js, only for elements inside a draggable element,
4. Drag an element from inside of draggable element to outside of it,
5. Drop an element on a placeholder,

Actual result:
Element is dropped on disallowed region.
Expected result:
Element should not be dropped on disallowed region.

The recording of incorrect behaviour: