WIP: Direct3D 11 Renderer

Implemented Direct3D 11 Renderer. On Windows systems it will be enabled by default. 

Potentially it should give you better performance and less CPU usage, especially with AMD graphics cards. Because the AMD OpenGL driver is not as good on Windows as it is on Linux.

For example, on an R5 5600g (with iGPU), I've got 50-75% less CPU usage with D3D11 than OGL.

Also, since SteamVR now uses a shared d3d11 texture instead of inter-op with OpenGL, VR performance could improve.