io/pointer: re-introduce PassOp

A previous change merged PassOp with AreaOp under the assumption that
the pass mode would be set on a particular area. That assumption turns
out not to hold, so this change brings back PassOp as an independent
stack operation.

This is an API change: replace AreaOp{Pass: true} with a separate
pointer.PassOp operation.

Fixes gio#288

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

widget/example_test.go

@@ -29,11 +29,9 @@ func ExampleClickable_passthrough() {
 	// widget lays out two buttons on top of each other.
 	widget := func() {
 		button1.Layout(gtx)
-		area := pointer.Rect(image.Rectangle{Max: gtx.Constraints.Max})
 		// button2 completely covers button1, but pass-through allows pointer
 		// events to pass through to button1.
-		area.PassThrough = true
-		defer area.Push(gtx.Ops).Pop()
+		defer pointer.PassOp{}.Push(gtx.Ops).Pop()
 		button2.Layout(gtx)
 	}
 

widget/material/list.go

@@ -161,7 +161,7 @@ func (s ScrollbarStyle) layout(gtx layout.Context, axis layout.Axis, viewportSta
 
 			// Stack a normal clickable area on top of the draggable area
 			// to capture non-dragging clicks.
-			pointerArea.PassThrough = true
+			defer pointer.PassOp{}.Push(gtx.Ops).Pop()
 			defer pointerArea.Push(gtx.Ops).Pop()
 			s.Scrollbar.AddTrack(gtx.Ops)
 
@@ -206,7 +206,7 @@ func (s ScrollbarStyle) layout(gtx layout.Context, axis layout.Axis, viewportSta
 
 				// Add the indicator pointer hit area.
 				area := pointer.Rect(image.Rectangle{Max: indicatorDims})
-				area.PassThrough = true
+				defer pointer.PassOp{}.Push(gtx.Ops).Pop()
 				defer area.Push(gtx.Ops).Pop()
 				s.Scrollbar.AddIndicator(gtx.Ops)