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.

NOTE: This PR uses [some definitions](https://github.com/dolphin-emu/dolphin/pull/1612/files#diff-6b31dc6e47429f9ea5b74738a8affb10R60) from the Nvidia SDK which falls under the [Nvidia SDK License](https://www.dropbox.com/s/i6ftg9wi9qyp5vz/License.pdf?dl=0). We should decide whether this code should be rewritten without the need for the definitions and whether that rewrite should be done through a clean room approach. However the definitions also include a magic number that we can't work around.

### To-do
- [x] Add anaglyph shader support.
- [x] Enable stereoscopy options.
It's only a cosmetic change, but the float constant "0" should be called "0.0". On GL, this would matter.