op/clip: split Rect into pixel-aligned Rect and rounded RRect

The pixel-aligned Rect is more efficient and easier to use in the common case
of layout clipping.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

layout/list.go

@@ -258,7 +258,7 @@ func (l *List) layout() Dimensions {
 			Max: axisPoint(l.Axis, max, inf),
 		}
 		stack := op.Push(ops)
-		clip.Rect{Rect: FRect(r)}.Add(ops)
+		clip.Rect(r).Add(ops)
 		op.Offset(FPt(axisPoint(l.Axis, pos, cross))).Add(ops)
 		child.call.Add(ops)
 		stack.Pop()