[42] wayland: Don't overwrite surface offsets

The intention when the offset request was added to protocol was that the attach request in a new enough protocol version should require dx/dy to be zero, but ignore them otherwise.

The current code checks for 0, but then overwrites the existing dx/dy with it, which renders an earlier wl_surface_offset() call ineffective.

Fixes: #2622 (closed) Part-of: !2843 (merged)

(cherry picked from commit ea373cb0)