test impl of "collision interpolator'; new number font for itemslots

src/net/torvald/spriteanimation/SpriteAnimation.kt

@@ -122,7 +122,7 @@ class SpriteAnimation(@Transient val parentActor: ActorWBMovable) {
      * @param scale
      */
     fun render(batch: SpriteBatch, posX: Float, posY: Float, scale: Float = 1f) {
-        if (cellWidth == 0 || cellHeight == 0) {
+        assert(cellWidth == 0 || cellHeight == 0) {
             throw Error("Sprite width or height is set to zero! ($cellWidth, $cellHeight); master: $parentActor")
         }