restorable: Refactoring

Reduces accesses to Image member from outside.

internal/graphicscommand/image.go

@@ -182,6 +182,11 @@ func (i *Image) IsInvalidated() bool {
 //
 // This is for testing usage.
 func (i *Image) Dump(path string) error {
+	// Screen image cannot be dumped.
+	if i.screen {
+		return nil
+	}
+
 	path = strings.ReplaceAll(path, "*", fmt.Sprintf("%d", i.id))
 	f, err := os.Create(path)
 	if err != nil {

internal/restorable/image.go

@@ -476,7 +476,11 @@ func (i *Image) hasDependency() bool {
 
 // Restore restores *graphicscommand.Image from the pixels using its state.
 func (i *Image) restore() error {
-	w, h := i.image.Size()
+	w, h := i.Size()
+
+	// Dispose the internal image after getting its size for safety.
+	i.image.Dispose()
+
 	if i.screen {
 		// The screen image should also be recreated because framebuffer might
 		// be changed.

internal/restorable/images.go

@@ -95,10 +95,7 @@ func RestoreIfNeeded() error {
 // This is for testing usage.
 func DumpImages(dir string) error {
 	for img := range theImages.images {
-		if img.screen {
-			continue
-		}
-		if err := img.image.Dump(filepath.Join(dir, "*.png")); err != nil {
+		if err := img.Dump(filepath.Join(dir, "*.png")); err != nil {
 			return err
 		}
 	}
@@ -154,12 +151,6 @@ func (i *images) restore() error {
 		panic("restorable: restore cannot be called when restoring is disabled")
 	}
 
-	// Dispose image explicitly
-	for img := range i.images {
-		img.image.Dispose()
-		// img.image can't be set nil here, or Size() panics when restoring.
-	}
-
 	// Let's do topological sort based on dependencies of drawing history.
 	// It is assured that there are not loops since cyclic drawing makes images stale.
 	type edge struct {