Blender Git Loki

Set up DirectX window to support drawing

With this the VR window should open fine and get cleared in a red-ish
orange using Direct3D 11 calls (well, on Windows that is).

The window still draws a 3D view to an offscreen buffer. Where we
usually just swap the buffers, we now allow calling a GHOST function to
blit the offscreen OpenGL buffer to whatever type of graphics buffer the
window uses (DirectX here).
The nice thing about this approach is that all DirectX code stays in
GHOST_ContextD3D.cpp. And the entire compatibiliy code can go into a
single function higher level modules don't need to care about.

This also fixes a number of issues introduced in earlier commits.