Examples + Viewport: GLFW: context wasn't set when using multiple windows. (#2392)

examples/imgui_impl_glfw.cpp

@@ -602,8 +602,11 @@ static void ImGui_ImplGlfw_RenderWindow(ImGuiViewport* viewport, void*)
 static void ImGui_ImplGlfw_SwapBuffers(ImGuiViewport* viewport, void*)
 {
     ImGuiViewportDataGlfw* data = (ImGuiViewportDataGlfw*)viewport->PlatformUserData;
-    if (g_ClientApi == GlfwClientApi_OpenGL)
+    if (g_ClientApi == GlfwClientApi_OpenGL) 
+    {
+        glfwMakeContextCurrent(data->Window);
         glfwSwapBuffers(data->Window);
+    }
 }
 
 //--------------------------------------------------------------------------------------------------------