Avoid non-initialized resize-sensor width/heights.

There is an issue when an element initially has zero width or height. The initialization does not set the resize-sensor-underflow to correctly pick up resize events in these cases. Additionally Firefox and Chrome seems to optimize away any underflow event to a div with no area.

The fix makes sure that the resize-sensors will get intializes correctly. It would be possible to initialize to 'null' instead, but then the comparisons afterwards needs to be a bit more strict.