VR: Refactor DirectX context management

All DirectX management happens on Ghost level now, higher level code can
just assume everything is OpenGL (except of the upside-down drawing that
still needs to be done for DirectX). This is similar to how the
metal-layer is hidden outside of Ghost.

The Ghost-XR graphics binding for DirectX is responsible for managing
the DirectX compatibility now.

intern/ghost/intern/GHOST_System.cpp

@@ -122,16 +122,6 @@ GHOST_TSuccess GHOST_System::disposeWindow(GHOST_IWindow *window)
   return success;
 }
 
-GHOST_IContext *GHOST_System::createOffscreenContext(GHOST_TDrawingContextType type)
-{
-  switch (type) {
-    case GHOST_kDrawingContextTypeOpenGL:
-      return createOffscreenContext();
-    default:
-      return NULL;
-  }
-}
-
 bool GHOST_System::validWindow(GHOST_IWindow *window)
 {
   return m_windowManager->getWindowFound(window);