Adjust cursor relative position when surface moves

Without this patch, keyboard doesn't toggle because the click and unclick events don't fall in the same spot. It's been applied upstream as well, so maybe it's easier to update librem5 to upstream.

Alternatively, this patch is unneeded once !19 (merged) and !20 (merged) get merged and #16 (closed) gets explained, to let upstream get merged into librem5.