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);
     }
 

imgui.cpp

@@ -3247,7 +3247,7 @@ void ImGui::NewFrame()
     {
         if ((g.IO.BackendFlags & ImGuiBackendFlags_PlatformHasViewports) && (g.IO.BackendFlags & ImGuiBackendFlags_RendererHasViewports))
         {
-            IM_ASSERT((g.FrameCount == 0 || g.FrameCount == g.FrameCountPlatformEnded) && "Forgot to call UpdatePlatformWindows() in main loop after EndFrame()?");
+            IM_ASSERT((g.FrameCount == 0 || g.FrameCount == g.FrameCountPlatformEnded) && "Forgot to call UpdatePlatformWindows() in main loop after EndFrame()? Check examples/ applications for reference.");
             IM_ASSERT(g.PlatformIO.Platform_CreateWindow != NULL  && "Platform init didn't install handlers?");
             IM_ASSERT(g.PlatformIO.Platform_DestroyWindow != NULL && "Platform init didn't install handlers?");
             IM_ASSERT(g.PlatformIO.Platform_GetWindowPos != NULL  && "Platform init didn't install handlers?");