get-pointer-position: use sheet-delta-transformation on graft's x/y
get-pointer-position: use sheet-delta-transformation on graft's x/y

Previous version used native transformation on parent's x/y. That is
fine until we try to pass here a sheet with a different mirror. Then
coordinates are wrong. This version works for any sheet being on the
same screen.

slyrus/mcclim,slyrus/mcclim