element.setSize doesn't play well with hdpi

[Marek Lewandowski]

This come out from #14630.

There as a failing test:

Error details for tests/plugins/maximize/maximize
test maximize fire resize event with proper properties
Width should be restored.
Expected: 200 (number)
Actual:   202 (number)

When I started to investigate what's the cause of this - it took me all the way to ​setSize method, and then ​marginAndPaddingSize function.

The problem there was that resize plugin was setting editor size, using box model so that border width should be included as a part of with. But it did not consider that the width might ba a fraction of a pixel, as a result in mentioned test border width was something like ~0.9px - but it used it as 0px. Having width attribute set to 200px (because border were skipped) and including the border, the actual width of the element was 202px.