Element.getSize() instead of Element.getComputedSize() in Mask.resize()

This fixes bad Mask resizing in case target element has a `box-sizing` that is not `content-box`. See #1273.

*PS: I don't know why Github is including old commits of mine in this PR, anyway they were already merged before 1.5.0 - and in fact they brings no diffs at all.*
`getComputedSize` does a lot of things, but one of them is that it checks for when the style is `auto`. Here you'd get `NaN` when you call `.toInt()`, I believe, if that's what the style is set to. It also will do this addition for you, telling you what the total margin is on the sides and the top & bottom. Is there a reason you opted to not use it?

If the purpose is to handle the box model stuff, I think all you need to do is change what `getComputedSize` measures, as you can configure it to use only the element size, the element size + margins, + margins and padding, + margins, padding, and border, etc.