restorable: Bug fix: Skip the screen image to check invalidation

internal/graphicscommand/image.go

@@ -154,6 +154,12 @@ func (i *Image) CopyPixels(src *Image) {
 }
 
 func (i *Image) IsInvalidated() bool {
+	if i.screen {
+		// The screen image might not have a texture, and in this case it is impossible to detect whether
+		// the image is invalidated or not.
+		panic("graphicscommand: IsInvalidated cannot be called on the screen image")
+	}
+
 	// i.image can be nil before initializing.
 	if i.image == nil {
 		return false

internal/restorable/images.go

@@ -72,6 +72,10 @@ func RestoreIfNeeded() error {
 		// As isInvalidated() is expensive, call this only for one image.
 		// This assumes that if there is one image that is invalidated, all images are invalidated.
 		for img := range theImages.images {
+			// The screen image might not have a texture. Skip this.
+			if img.screen {
+				continue
+			}
 			r = img.isInvalidated()
 			break
 		}