ui/pointer: introduce OpArea for pointer hit testing

Split out OpArea from OpHandler to allow stacked areas in a followup.

Replace hit closures with static shapes (rectangles and ellipses) to
avoid allocations. If needed, generic hit functions can be introduced
again later.

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

ui/text/editor.go

@@ -201,9 +201,9 @@ func (e *Editor) Layout(ops *ui.Ops, cs layout.Constraints) layout.Dimens {
 	}
 
 	baseline := e.padTop + e.dims.Baseline
-	area := &gesture.Rect{e.viewSize}
-	e.scroller.Op(ops, area)
-	e.clicker.Op(ops, area)
+	pointer.AreaRect(e.viewSize).Add(ops)
+	e.scroller.Add(ops)
+	e.clicker.Add(ops)
 	return layout.Dimens{Size: e.viewSize, Baseline: baseline}
 }