op: rename StackOp/Push/Pop to StateOp/Save/Load

The semantics were relaxed in a previous commit; this change renames
to operations accordingly.

API change. Use gofmt to adjust your code accordingly:

gofmt -r 'op.Push(a).Pop() -> op.Save(a).Load()'
gofmt -r 'op.Push(a) -> op.Save(a)'
gofmt -r 'v.Pop() -> v.Load()'
gofmt -r 'op.StackOp -> op.StateOp'

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

widget/float.go

@@ -52,7 +52,7 @@ func (f *Float) Layout(gtx layout.Context, pointerMargin int, min, max float32)
 		f.pos = 1
 	}
 
-	defer op.Push(gtx.Ops).Pop()
+	defer op.Save(gtx.Ops).Load()
 	rect := image.Rectangle{Max: size}
 	rect.Min.X -= pointerMargin
 	rect.Max.X += pointerMargin