restorable: Bug fix: dummyImage must be restored first

Fixes #791

internal/restorable/image.go

@@ -55,6 +55,9 @@ type Image struct {
 
 	w2 int
 	h2 int
+
+	// priority indicates whether the image is restored in high priority when context-lost happens.
+	priority bool
 }
 
 var dummyImage *Image
@@ -62,6 +65,7 @@ var dummyImage *Image
 func init() {
 	dummyImage = &Image{
 		image:    graphicscommand.NewImage(16, 16),
+		priority: true,
 	}
 	theImages.add(dummyImage)
 }
@@ -98,11 +102,7 @@ func NewScreenFramebufferImage(width, height int) *Image {
 
 func (i *Image) clear() {
 	// There are not 'drawImageHistoryItem's for this image and dummyImage.
-	// This means dummyImage might not be restored yet when this image is restored.
-	// However, that's ok since this image will be stale or have its updated pixel data soon,
-	// and this image can be restored without dummyImage.
-	//
-	// dummyImage should be restored later anyway.
+	// As dummyImage is a priority image, this is restored faster than other regular images.
 	w, h := i.Size()
 	sw, sh := dummyImage.Size()
 	dw := graphics.NextPowerOf2Int(w)

internal/restorable/images.go

@@ -164,15 +164,23 @@ func (i *images) restore() error {
 	}
 	images := map[*Image]struct{}{}
 	for i := range i.images {
+		if !i.priority {
 			images[i] = struct{}{}
 		}
+	}
 	edges := map[edge]struct{}{}
 	for t := range images {
 		for s := range t.dependingImages() {
 			edges[edge{source: s, target: t}] = struct{}{}
 		}
 	}
+
 	sorted := []*Image{}
+	for i := range i.images {
+		if i.priority {
+			sorted = append(sorted, i)
+		}
+	}
 	for len(images) > 0 {
 		// current repesents images that have no incoming edges.
 		current := map[*Image]struct{}{}
@@ -198,6 +206,7 @@ func (i *images) restore() error {
 			delete(edges, e)
 		}
 	}
+
 	for _, img := range sorted {
 		if err := img.restore(); err != nil {
 			return err