io/router: deliver synthetic events to sibling pointer handlers

Before this change, synthetic events such as scrolling caused by
focus movement would use semantic information to determine potential
receivers. However, there can only be one handler per area so sibling
handlers would not be considered. This change makes the event delivery
traverse the entire tree of handlers, including siblings.

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

io/router/key_test.go

@@ -282,6 +282,10 @@ func TestFocusScroll(t *testing.T) {
 		Types:        pointer.Scroll,
 		ScrollBounds: image.Rect(-100, -100, 100, 100),
 	}.Add(ops)
+	// Test that h is scrolled even if behind another handler.
+	pointer.InputOp{
+		Tag: new(int),
+	}.Add(ops)
 	cl.Pop()
 	parent.Pop()
 	r.Frame(ops)