Build Testing With DX12 Reserved For Windows 10

[vlj]

Please note that the backend is barely 2 weeks old and there are a lot of things not supported :

* indexed drawing
* multi texture
* "complex" primitive (like dotted line, polygon mode,...) but I'm not sure these features are widely used anyway.
* Stencil
* Discard
* Non bilinear sampling

Blending is also quite basic.
However all static tests from auto test repo seems to works correctly, as well as dynamic test 1 and 2.
Depth read works (looks like it's broken for GL, will have to look at it).

You can select the "WARP" adaptater to use Microsoft software rendering.
So far the backend works correctly with radeon, on Haswell it doesn't crash but the windows are empty and on Nvidia there is a crash (maybe fixed with latest commit).

Performance-wise D3D12 backend is currently slower than GL renderer due to depth read, I have to optimise it a little more.