add get svg function

src/virtual-machine.js

@@ -340,6 +340,20 @@ class VirtualMachine extends EventEmitter {
         return null;
     }
 
+    /**
+     * Get an SVG string from the renderer.
+     * @param {int} costumeIndex - the index of the sound to be got.
+     * @return {string} the costume's SVG string, or null if it's not an SVG costume.
+     */
+    getCostumeSVG (costumeIndex) {
+        const id = this.editingTarget.sprite.costumes[costumeIndex].assetId;
+        if (id && this.runtime && this.runtime.storage &&
+                this.runtime.storage.get(id).dataFormat === 'svg') {
+            return this.runtime.storage.get(id).decodeText();
+        }
+        return null;
+    }
+
     /**
      * Update a sound buffer.
      * @param {int} soundIndex - the index of the sound to be updated.