Poll all windows in the multi window sample

On Linux all windows are polled separately contrary to windows in which
the callback is used on all windows.

BUG=angleproject:892

Change-Id: Ib1538f12ab2fa1864d29c91b9c5bfc2204cd1fa7
Reviewed-on: https://chromium-review.googlesource.com/272945
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>

samples/multi_window/MultiWindow.cpp

@@ -99,6 +99,11 @@ class MultiWindowSample : public SampleApplication
     virtual void step(float dt, double totalTime)
     {
         mRotation = fmod(mRotation + (dt * 40.0f), 360.0f);
+
+        for (size_t i = 1; i < mWindows.size(); i++)
+        {
+            mWindows[i].osWindow->messageLoop();
+        }
     }
 
     virtual void draw()