cursor: offset xdg invisible border when dragging outside view

cursor: offset xdg invisible border when dragging outside view

Note: view->padding was deleted in commit b279550 as the wlroots
scene-graph xdg-surface commit handler offsets the CSD invisible border.
If view->padding still existed, we could have used that, but considering
that this is probably the only place where this offset will now be
needed (because we're generating surface local coordinates), it's simpler
to just do it locally.

Related to issue #340

Written-by: @Consolatis