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>

op/paint/paint.go

@@ -139,7 +139,7 @@ func (d PaintOp) Add(o *op.Ops) {
 
 // FillShape fills the clip shape with a color.
 func FillShape(ops *op.Ops, c color.NRGBA, shape clip.Op) {
-	defer op.Push(ops).Pop()
+	defer op.Save(ops).Load()
 	shape.Add(ops)
 	Fill(ops, c)
 }
@@ -149,7 +149,7 @@ func FillShape(ops *op.Ops, c color.NRGBA, shape clip.Op) {
 // the painted area. Use FillShape unless you need to paint several
 // times within the same clip.Op.
 func Fill(ops *op.Ops, c color.NRGBA) {
-	defer op.Push(ops).Pop()
+	defer op.Save(ops).Load()
 	ColorOp{Color: c}.Add(ops)
 	PaintOp{}.Add(ops)
 }