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/material/loader.go

@@ -36,7 +36,7 @@ func (l LoaderStyle) Layout(gtx layout.Context) layout.Dimensions {
 	}
 	sz := gtx.Constraints.Constrain(image.Pt(diam, diam))
 	radius := float64(sz.X) * .5
-	defer op.Push(gtx.Ops).Pop()
+	defer op.Save(gtx.Ops).Load()
 	op.Offset(f32.Pt(float32(radius), float32(radius))).Add(gtx.Ops)
 
 	dt := (time.Duration(gtx.Now.UnixNano()) % (time.Second)).Seconds()