Eager load used sprites

internal/assetstore/object.go

@@ -1,6 +1,7 @@
 package assetstore
 
 import (
+	"fmt"
 	"log"
 
 	"github.com/hajimehoshi/ebiten"
@@ -18,6 +19,7 @@ type Object struct {
 type Sprite struct {
 	obj *Object
 
+	ID      string
 	XOffset int
 	YOffset int
 	Width   int
@@ -50,8 +52,8 @@ func (a *AssetStore) Object(name string) (*Object, error) {
 		sprites: make([]*Sprite, int(raw.NumSprites)),
 		raw:     raw,
 	}
-	a.objs[name] = obj
 
+	a.objs[name] = obj
 	return obj, nil
 }
 
@@ -75,6 +77,7 @@ func (o *Object) Sprite(idx int) (*Sprite, error) {
 	}
 
 	sprite := &Sprite{
+		ID:      fmt.Sprintf("%v:%v", o.raw.Name, idx),
 		obj:     o,
 		Width:   int(raw.Width),
 		Height:  int(raw.Width),