Minor: fix GLFW warnings when destroying the window

Summary:
The code was calling glfwTerminate() before destroying the window.

I wanted to do the same for the Windows+Angle shell apps, but they aren't working for me (GL_INVALID_OPERATION on every GL call).

Differential Revision: D50761801

fbshipit-source-id: ffc23ab70d5d7ceafd35a8a44bb69ba3581befb8

shell/windows/opengl/App.cpp

@@ -231,6 +231,11 @@ static void RunApplicationMode(uint32_t majorVersion, uint32_t minorVersion) {
     }
   }
 
+  glShellPlatform_ = nullptr;
+  glDevice = nullptr;
+  context = nullptr;
+  glWindow = nullptr;
+
   glfwTerminate();
 }