To enable the Debug Layer, set the hint, SDL_HINT_RENDER_DIRECT3D11_DEBUG to '1'.
The Debug Layer will be turned off by default, both in Release and Debug builds (of SDL).
This change should allow apps to render correctly in Portrait mode, at minimum,
Support for orientation changes is pending.
Thanks to Pierre-Yves for assistance!
This is part of an overall effort to use the name, "WinRT", rather than "WindowsRT" (or "Windows RT"), as the shorthand name often seems to mean something different than the longhand name. (WinRT is an API, Windows RT is a product name)