restorable: Add ClearPixels

This hides the implementation details of allocating byte slice.

This change also adds comments about #897.

internal/restorable/image.go

@@ -281,6 +281,12 @@ func (i *Image) CopyPixels(src *Image) {
 	i.makeStale()
 }
 
+// ClearPixels clears the specified region by ReplacePixels.
+func (i *Image) ClearPixels(x, y, width, height int) {
+	// TODO: Allocating bytes for all pixels are wasteful. Allocate memory only for required regions (#897).
+	i.ReplacePixels(make([]byte, 4*width*height), x, y, width, height)
+}
+
 // ReplacePixels replaces the image pixels with the given pixels slice.
 //
 // If pixels is nil, ReplacePixels clears the specified reagion.
@@ -298,6 +304,8 @@ func (i *Image) ReplacePixels(pixels []byte, x, y, width, height int) {
 	theImages.makeStaleIfDependingOn(i)
 
 	if pixels == nil {
+		// TODO: Allocating bytes for all pixels are wasteful. Allocate memory only for required regions
+		// (#897).
 		pixels = make([]byte, 4*width*height)
 	}
 	i.image.ReplacePixels(pixels, x, y, width, height)

internal/shareable/shareable.go

@@ -400,9 +400,7 @@ func (i *Image) dispose(markDisposed bool) {
 	i.backend.page.Free(i.node)
 	if !i.backend.page.IsEmpty() {
 		// As this part can be reused, this should be cleared explicitly.
-		x, y, w, h := i.region()
+		i.backend.restorable.ClearPixels(i.region())
-		// TODO: Now nil cannot be used here (see the test result). Fix this.
-		i.backend.restorable.ReplacePixels(make([]byte, 4*w*h), x, y, w, h)
 		return
 	}