op: tolerate incomplete macros

Before this change, a macro not Stop'ed would result in an endless
loop during op decoding.

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

op/op_test.go

@@ -5,6 +5,8 @@ package op
 import (
 	"image"
 	"testing"
+
+	"gioui.org/internal/ops"
 )
 
 func TestTransformChecks(t *testing.T) {
@@ -18,3 +20,17 @@ func TestTransformChecks(t *testing.T) {
 	Record(&ops)
 	trans.Pop()
 }
+
+func TestIncompleteMacroReader(t *testing.T) {
+	var o Ops
+	// Record, but don't Stop it.
+	Record(&o)
+	Offset(image.Point{}).Push(&o)
+
+	var r ops.Reader
+
+	r.Reset(&o.Internal)
+	if _, more := r.Decode(); more {
+		t.Error("decoded an operation from a semantically empty Ops")
+	}
+}