Document and test that saveProjectSb3DontZip does not copy asset buffers

src/virtual-machine.js

@@ -524,7 +524,9 @@ class VirtualMachine extends EventEmitter {
 
     /**
      * tw: Serialize the project into a map of files without actually zipping the project.
-     * @returns {Record<Uint8Array>} Files of the project.
+     * The buffers returned are the exact same ones used internally, not copies. Avoid directly
+     * manipulating them (except project.json, which is created by this function).
+     * @returns {Record<string, Uint8Array>} Map of file name to the raw data for that file.
      */
     saveProjectSb3DontZip () {
         const soundDescs = serializeSounds(this.runtime);

test/integration/tw_save_project_sb3.js

@@ -78,5 +78,9 @@ test('saveProjectSb3DontZip', async t => {
     t.equal(map['d9c625ae1996b615a146ac2a7dbe74d7.svg'].byteLength, 691);
     t.equal(map['cd21514d0531fdffb22204e0ec5ed84a.svg'].byteLength, 202);
 
+    // Make sure that the asset buffers returned are the exact same as the ones used internally, not copies.
+    const costume = vm.runtime.targets[0].getCostumes()[0];
+    t.equal(map['cd21514d0531fdffb22204e0ec5ed84a.svg'], costume.asset.data);
+
     t.end();
 });