internal/ui: bug fix: SetWindowIcon(nil) didn't reset the window icon

Closes #2796

examples/windowsize/main.go

@@ -307,6 +307,9 @@ func (g *game) Update() error {
 	if inpututil.IsKeyJustPressed(ebiten.KeyI) {
 		ebiten.SetWindowIcon([]image.Image{createRandomIconImage()})
 	}
+	if inpututil.IsKeyJustPressed(ebiten.KeyJ) {
+		ebiten.SetWindowIcon(nil)
+	}
 
 	ebiten.SetWindowMousePassthrough(mousePassthrough)
 
@@ -358,6 +361,7 @@ func (g *game) Draw(screen *ebiten.Image) {
 [U] Switch the runnable-on-unfocused state
 [C] Switch the cursor mode (visible, hidden, or captured)
 [I] Change the window icon (only for desktops)
+[J] Reset the window icon (only for desktops)
 [V] Switch the vsync
 [T] Switch TPS (ticks per second)
 [D] Switch the window decoration (only for desktops)

internal/goglfw/win32window_windows.go

@@ -1559,6 +1559,9 @@ func (w *Window) platformSetWindowIcon(images []*Image) error {
 	if len(images) > 0 {
 		w.platform.bigIcon = bigIcon
 		w.platform.smallIcon = smallIcon
+	} else {
+		w.platform.bigIcon = 0
+		w.platform.smallIcon = 0
 	}
 	return nil
 }

internal/ui/ui_glfw.go

@@ -1204,11 +1204,17 @@ func (u *userInterfaceImpl) updateIconIfNeeded() error {
 	}
 
 	imgs := u.getAndResetIconImages()
-	if len(imgs) == 0 {
+	// A 0-size slice and nil are distinguished here.
+	// A 0-size slice means a user indicates to reset the icon.
+	// On the other hand, nil means a user didn't update the icon state.
+	if imgs == nil {
 		return nil
 	}
 
-	newImgs := make([]image.Image, len(imgs))
+	var newImgs []image.Image
+	if len(imgs) > 0 {
+		newImgs = make([]image.Image, len(imgs))
+	}
 	for i, img := range imgs {
 		// TODO: If img is not *ebiten.Image, this converting is not necessary.
 		// However, this package cannot refer *ebiten.Image due to the package