Viewport: Fixed SDL+OpenGL2 example to work with multi-viewports. (#2175)

examples/example_sdl_opengl2/main.cpp

@@ -135,6 +135,15 @@ int main(int, char**)
         glClear(GL_COLOR_BUFFER_BIT);
         //glUseProgram(0); // You may want this if using this code in an OpenGL 3+ context where shaders may be bound
         ImGui_ImplOpenGL2_RenderDrawData(ImGui::GetDrawData());
+
+        // Update and Render additional Platform Windows
+        if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
+        {
+            ImGui::UpdatePlatformWindows();
+            ImGui::RenderPlatformWindowsDefault();
+        }
+
+        SDL_GL_MakeCurrent(window, gl_context);
         SDL_GL_SwapWindow(window);
     }