Make animations work in the options screen

internal/ui/animation.go

@@ -0,0 +1,15 @@
+package ui
+
+import (
+	"github.com/hajimehoshi/ebiten"
+)
+
+type animation []*ebiten.Image
+
+func (a animation) image(step int) *ebiten.Image {
+	if len(a) == 0 {
+		return nil
+	}
+
+	return a[step%len(a)]
+}