screenedge: don't reduce approachGeometry

Reducing approachGeometry is not needed.

During edge creation, createHorizontalEdge and createVerticalEdge
already substract the needed cornerOffset, and edges reserved by client
windows shouldn't be reduced in geometry.
In addition, during display, ScreenEdgeEffect confines the painted area
to approachGeometry. So approachGeometry shouldn't be reduced in
advance.

BUG: 481282


(cherry picked from commit 2f4db693)