op/paint,io/system: document ImageOp lifetime

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

io/system/system.go

@@ -23,6 +23,25 @@ type FrameEvent struct {
 	Insets Insets
 	// Frame is the callback to supply the list of
 	// operations to complete the FrameEvent.
+	//
+	// Note that the operation list and the operations themselves
+	// may not be mutated until another FrameEvent is received from
+	// the same event source.
+	// That means that calls to frame.Reset and changes to referenced
+	// data such as ImageOp backing images should happen between
+	// receiving a FrameEvent and calling Frame.
+	//
+	// Example:
+	//
+	//	var w *app.Window
+	//	var frame *op.Ops
+	// 	for e := range w.Events() {
+	//  	if e, ok := e.(system.FrameEvent); ok {
+	//			// Call frame.Reset and manipulate images for ImageOps
+	//			// here.
+	//			e.Frame(frame)
+	//  	}
+	//  }
 	Frame func(frame *op.Ops)
 	// Queue supplies the events for event handlers.
 	Queue event.Queue