function getFirstCollision(layout
/*: Layout*/
, layoutItem
/*: LayoutItem*/
)
/*: ?LayoutItem*/
{
  for (var i = 0, len = layout.length; i < len; i++) {
    if (collides(layout[i], layoutItem)) return layout[i];
  }
}
Returns the first item this layout collides with.
It doesn't appear to matter which order we approach this from, although
perhaps that is the wrong thing to do.

@param  {Object} layoutItem Layout item.
@return {Object|undefined}  A colliding layout item, or undefined.