wayland/dnd-surface: Apply surface offset

From the second commit:

    The surface offset allows an application to move itself in relative
    coordinates to its previous position. It is rather ill defined and
    partly incompatible with other functionality, which is why we ignore
    it generally.
    
    For dnd-surfaces though, it is the de-facto standard for applications
    to properly position the dnd-icon below the cursor. Therefore apply
    the offset on actor sync.

Fixes #679 (closed)

Tested with nautilus, weston-dnd and firefox (on X11) with and without (fractional) scaling. Firefox on Wayland with fractional scaling does net yet behave quite right, but I'm pretty sure that's a Firefox specific problem. When testing, have #685 (closed) in mind.

For Firefox/Thunderbird on HIDPI see https://bugzilla.mozilla.org/show_bug.cgi?id=1567463