wayland: Keep hotspot when changing cursors

wayland: Keep hotspot when changing cursors

We need to pass the delta between the old and new hotspot
when attaching the new cursor surface, to keep the hotspot
at the same position. We can't deal with this in the compositor,
since the set_cursor call already overwrites the old hotspot,
so the information is lost by the time the attach happens.
Unfortunately, we can't query the initial hotspot from
the compositor, so the first cursor change will make the
hotspot jump.
https://bugzilla.gnome.org/show_bug.cgi?id=695512