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>

internal/rendertest/transform_test.go

@@ -130,7 +130,7 @@ func TestOffsetScaleTexture(t *testing.T) {
 
 func TestRotateTexture(t *testing.T) {
 	run(t, func(o *op.Ops) {
-		defer op.Push(o).Pop()
+		defer op.Save(o).Load()
 		squares.Add(o)
 		a := f32.Affine2D{}.Offset(f32.Pt(30, 30)).Rotate(f32.Pt(40, 40), math.Pi/4)
 		op.Affine(a).Add(o)