Calculate rotation center if it's not supplied

When a new skin is added, previously the rotation center was set to `[0, 0]`.  In the case of costumes and backdrops added from libraries, the center should be calculated from the bounding box of the imported skin.

Toward LLK/scratch-gui#18

src/Skin.js

@@ -83,6 +83,14 @@ class Skin extends EventEmitter {
         }
     }
 
+    /**
+     * Get the center of the current bounding box
+     * @return {[number,number]} the center of the current bounding box
+     */
+    calculateRotationCenter () {
+        return [this.size[0] / 2, this.size[1] / 2];
+    }
+
     /**
      * @abstract
      * @param {[number,number]} scale - The scaling factors to be used.