[droppable] Defer querying of element proportions until necessary

[Reading offsetWidth and offsetHeight triggers a synchronous reflow](http://calendar.perfplanet.com/2009/rendering-repaint-reflow-relayout-restyle/) – an event that causes the browser to calculate the layout of the page at that given moment so that it can be sure of the value you've asked for.

This commit defers the querying of those properties until they're actually required, then memoizes the result as before, eliminating superfluous reflows as droppables are created.