compositing: Don't count the root layer when checking to see when to take the screenshot.

components/compositing/compositor.rs

@@ -1324,7 +1324,12 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         // have not requested a paint of the current epoch.
         // If a layer has sent a request for the current epoch, but it hasn't
         // arrived yet then this layer is waiting for a paint message.
-        if layer_data.requested_epoch == current_epoch && layer_data.painted_epoch != current_epoch {
+        //
+        // Also don't check the root layer, because the paint task won't paint
+        // anything for it after first layout.
+        if layer_data.id != LayerId::null() &&
+                layer_data.requested_epoch == current_epoch &&
+                layer_data.painted_epoch != current_epoch {
             return true;
         }