Avoid broadcasting what was just received (#1116)

Fixes #1115

The issue is that replaceAllElements calls a render synchronously, preventing lastBroadcastedOrReceivedSceneVersion from being set correctly.

I tried using batchUpdate but it only takes a single argument ( c5d2fc7127/packages/react-reconciler/src/ReactFiberWorkLoop.js (L1088) ) whereas the callback takes two.

Test Plan:
- Add a console.log before `this.broadcastScene("SCENE_UPDATE");` in App.tsx
- Connect a bunch of clients
- Have one move a shape
- Make sure that this client has the console logged
- Make sure the other clients don't have it

src/components/App.tsx

@@ -731,8 +731,7 @@ export class App extends React.Component<any, AppState> {
           );
 
           // Reconcile
-          globalSceneState.replaceAllElements(
-            restoredState.elements
+          const newElements = restoredState.elements
             .reduce((elements, element) => {
               // if the remote element references one that's currently
               //  edited on local, skip it (it'll be added in the next
@@ -777,13 +776,19 @@ export class App extends React.Component<any, AppState> {
               return elements;
             }, [] as Mutable<typeof restoredState.elements>)
             // add local elements that weren't deleted or on remote
-              .concat(...Object.values(localElementMap)),
-          );
-        }
+            .concat(...Object.values(localElementMap));
+
+          // Avoid broadcasting to the rest of the collaborators the scene
+          // we just received!
+          // Note: this needs to be set before replaceAllElements as it
+          // syncronously calls render.
           this.lastBroadcastedOrReceivedSceneVersion = getDrawingVersion(
-          globalSceneState.getAllElements(),
+            newElements,
           );
 
+          globalSceneState.replaceAllElements(newElements);
+        }
+
         // We haven't yet implemented multiplayer undo functionality, so we clear the undo stack
         // when we receive any messages from another peer. This UX can be pretty rough -- if you
         // undo, a user makes a change, and then try to redo, your element(s) will be lost. However,