Protect against generation of no-change viewport resize events. (#454)

I noticed 8 or 10 of these events being generated in a multi-tab browsing session on Chrome 87.0 on Win10.  I'm speculating they were generated as a side effect of changing tabs but I can't recreate

src/record/observer.ts

@@ -215,13 +215,19 @@ function initScrollObserver(
 function initViewportResizeObserver(
   cb: viewportResizeCallback,
 ): listenerHandler {
+  let last_h = -1;
+  let last_w = -1;
   const updateDimension = throttle(() => {
     const height = getWindowHeight();
     const width = getWindowWidth();
+    if (last_h !== height || last_w != width) {
       cb({
         width: Number(width),
         height: Number(height),
       });
+      last_h = height;
+      last_w = width;
+    }
   }, 200);
   return on('resize', updateDimension, window);
 }