Fix ElementHandle resizing incorrectly when the size_target has "box-sizing: border-box"

[nimble0]

This is an example of the issue. When the drag starts it jumps so that the cursor is below the blue box instead of in the middle where it started.

[mikke89]

Thanks for the PR!

One issue with the proposed solution is that not all elements specify their width and height properties. Eg. when one uses top/left/bottom/right properties on a window body, which is a quite common case.

I think a better fix is to get the target's computed box sizing property GetComputedValues().box_sizing and select between the formatted box's content size and border size (Box::CONTENT and Box::BORDER) accordingly, if that makes sense.

[nimble0]

That makes sense, I've updated it to do that.

[mikke89]

Perfect, thank you!