D3D: Add geometry shader stereoscopy support.

Continuation of PR #1439. Like the OGL backend MSAA and XFB are not yet supported.

Besides the usual stereoscopy modes the D3D also offers support for Nvidia 3D Vision. Allowing you to use the native stereoscopy with the 3D Vision platform.

### To-do
- [x] Add anaglyph shader support.
- [ ] Enable stereoscopy options.
Well spotted, I forgot to mention it in the description, I discussed this with degasus before. He recommended that I wrote an implementation that didn't rely on the struct, but Nvidia 3D Vision is difficult to get working correctly so I decided to copy it just to be sure.

The struct stems from `nvstereo.h` which falls under the [Nvidia SDK](https://www.dropbox.com/s/i6ftg9wi9qyp5vz/License.pdf?dl=0) license. Not sure yet what to do about it, I could easily write an implementation that doesn't rely on the struct.