ui: make OpPush and OpPop explicit

We're about to allow OpBlock for invoking ops from multiple (cached)
Ops containers. To allow for drawing state changes to stick after
invoking such a cached block, we can't let OpBlock perform an implicit
save and restore of drawing state.

Instead, introduce OpPush and OpPop for explicit drawing state stack
management.

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

ui/text/editor.go

@@ -171,11 +171,11 @@ func (e *Editor) Layout(ops *ui.Ops, cs layout.Constraints) layout.Dimens {
 			break
 		}
 		path := e.Face.Path(str)
-		ops.Begin()
+		ui.OpPush{}.Add(ops)
 		ui.OpTransform{Transform: ui.Offset(lineOff)}.Add(ops)
 		draw.OpClip{Path: path}.Add(ops)
 		draw.OpDraw{Rect: toRectF(clip).Sub(lineOff)}.Add(ops)
-		ops.End().Add(ops)
+		ui.OpPop{}.Add(ops)
 	}
 	if e.focused {
 		now := e.cfg.Now