clip: check Path ops are not mixed with others

Multiple operations Op, such as clip.Path, cannot
be interleaved with other ops. This patch adds a
mechanism to ensure that is the case by starting
multi ops with ops.BeginMulti and ending them with
ops.EndMulti while operations are written to op.Ops
with ops.WriteMulti.
This mechanism is applied to clip.Path.

Fixes: https://todo.sr.ht/~eliasnaur/gio/336
Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

internal/ops/ops.go

@@ -22,6 +22,8 @@ type Ops struct {
 	// nextStateID is the id allocated for the next
 	// StateOp.
 	nextStateID int
+	// multipOp indicates a multi-op such as clip.Path is being added.
+	multipOp bool
 
 	macroStack stack
 	stacks     [5]stack
@@ -192,6 +194,31 @@ func Reset(o *Ops) {
 }
 
 func Write(o *Ops, n int) []byte {
+	if o.multipOp {
+		panic("cannot mix multi ops with single ones")
+	}
+	o.data = append(o.data, make([]byte, n)...)
+	return o.data[len(o.data)-n:]
+}
+
+func BeginMulti(o *Ops) {
+	if o.multipOp {
+		panic("cannot interleave multi ops")
+	}
+	o.multipOp = true
+}
+
+func EndMulti(o *Ops) {
+	if !o.multipOp {
+		panic("cannot end non multi ops")
+	}
+	o.multipOp = false
+}
+
+func WriteMulti(o *Ops, n int) []byte {
+	if !o.multipOp {
+		panic("cannot use multi ops in single ops")
+	}
 	o.data = append(o.data, make([]byte, n)...)
 	return o.data[len(o.data)-n:]
 }