fix - needed viewport sizes update

viewport sizes are now updated the first time a DOM element is registered as autoresizable. 
After this initialization, all the resize events will trigger the ```__applyNewSize()``` method (as before the change).

Instead, before this change, the sizes were not up to date until a resize event was raised so, for instance, if the window was resized before the first autoresizable element was introduced, the viewport change was not taken into account to draw the element.