Account for scaling when computing element dimensions

[niklasramo]

All the methods that need to use getComputedStyle(el) are not working as expected when the element is scaled (e.g. transform: scale(0.5)). This is because the getComputedStyle(el) doesn't account for scaling unlike getBoundingClientRect().

In practise we need a way to compute scaled paddings, borders, margins and scrollbar size. Since doing that computation might potentially cause quite a lot of extra work maybe it's better to add an option to query the dimensions which accounts for transforms.