Break load costume into more functions, and update how bitmaps are loaded

src/engine/runtime.js

@@ -995,6 +995,15 @@ class Runtime extends EventEmitter {
         this.v2SvgAdapter = svgAdapter;
     }
 
+    /**
+     * Set the bitmap adapter for the VM/runtime, which converts scratch 2
+     * bitmaps to scratch 3 bitmaps. (Scratch 3 bitmaps are all bitmap resolution 2)
+     * @param {!function} bitmapAdapter The adapter to attach
+     */
+    attachV2BitmapAdapter (bitmapAdapter) {
+        this.v2BitmapAdapter = bitmapAdapter;
+    }
+
     /**
      * Attach the storage module
      * @param {!ScratchStorage} storage The storage module to attach