CSS: Make `offsetHeight( true )`, etc. include negative margins

CSS: Make `offsetHeight( true )`, etc. include negative margins

This regressed in gh-3656 as the added logic to include scroll gutters
in `.innerWidth()` / `.innerHeight()` didn't take negative margins into
account. This broke handling of negative margins in
`.offsetHeight( true )` and `.offsetWidth( true )`. To fix it, calculate
margin delta separately and only add it after the scroll gutter
adjustment logic.

Fixes gh-3982
Closes gh-5234
Ref gh-3656

(cherry picked from commit bce13b72c1753e16cc0db53ebf0f0456bdcf6b48)