op: implement StackOp in terms of general save/load of state

Push/Pop only allows saving and restoring operation state in a
stack-like manner. We're going to need restoring arbitrary state
for implementing deferred operations.

Generalize state save/restore and implement Push and Pop on top of
that.

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

internal/ops/ops.go

@@ -62,3 +62,21 @@ func DecodeTransform(data []byte) (t f32.Affine2D) {
 	f := math.Float32frombits(bo.Uint32(data[4*5:]))
 	return f32.NewAffine2D(a, b, c, d, e, f)
 }
+
+// DecodeSave decodes the state id of a save op.
+func DecodeSave(data []byte) int {
+	if opconst.OpType(data[0]) != opconst.TypeSave {
+		panic("invalid op")
+	}
+	bo := binary.LittleEndian
+	return int(bo.Uint32(data[1:]))
+}
+
+// DecodeLoad decodes the state id of a restore op.
+func DecodeLoad(data []byte) int {
+	if opconst.OpType(data[0]) != opconst.TypeLoad {
+		panic("invalid op")
+	}
+	bo := binary.LittleEndian
+	return int(bo.Uint32(data[1:]))
+}