Add DirectX 12.x support. This is windows (and later, Xbox) only:
Core features:

• DX12 - Device Detection #159
• DX12 - Device Initialization #160
• DX12 - Monitor/output detection #161
• DX12 - Swap-chain initialization #162
• DX12 - Windowing Support #163
• DX12 - Resource base types #164
• DX12 - Buffer resources #165
• DX12 - Command queue management #166
• DX12 - GPU State management #167
• DX12 - HLSL shader compilation #168
• DX12 - Texture1D #169
• DX12 - Texture2D #170
• DX12 - Texture3D #171
• DX12 - Render Surfaces #172
• DX12 - Texture array support #173
• DX12 - Compute tasking - Use a separate compute command queue for tasking purposes #174
• DX12 - Multi-GPU support - Use secondary GPUs to offload compute or geometry streaming tasks #175