Allow changing sprite ctor

Engine/src/shared/sprites.ts

@@ -10,19 +10,26 @@ for (let [key, value] of Object.entries(assets)) {
 
 export class SpriteInstance {
   container: PIXI.Container
-  sprite: Sprite
+  sprite: Sprite|null = null
+  spriteKey: string = ''
   animation?: SpritePart
-  animationKey: string
+  animationKey: string = ''
   set?: SpritePart
-  setKey: string
+  setKey: string = ''
   subset?: SpritePart
-  subsetKey: string
+  subsetKey: string = ''
   frame?: SpritePart
-  frameIndex: number
+  frameIndex: number = 0
 
   constructor(ctor: string) {
     this.container = new PIXI.Container()
+    this.setCtor(ctor)
+  }
+
+  setCtor(ctor: string) {
+    this.container.removeChildren()
     let [spriteKey, animationKey, setKey, subsetKey, frameKey] = ctor.split('.')
+    this.spriteKey = spriteKey
     this.animationKey = animationKey
     this.setKey = setKey
     this.subsetKey = subsetKey