Fix item positioning on grab.
Fix item positioning on grab.

We were invoking grabItem too early. grabItem redraws the element
already absolutely positioned, which messes with offset measuring.
It was causing items to "jump" to the left a few pixels after grabbed.

edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2