DirectCompute support

[walbourn]

Similar to the IEffect interface and the built-in shaders, we could create a set of DirectCompute shaders as well--which is easier done with DirectX Tool Kit for DX12 since all supported feature levels can do DirectCompute 5.0 or better.

Possible built-in compute shaders:

• AdaptExposure
• Ambient occlusion
• Average luminance
• Bitonic sort
• Bloom
• Blur
• Depth-of-field
• FXAA
• Histogram
• Motion blur

A number of these fall into the existing PostProcess class, but would do it via DirectCompute instead of a Pixel shader.

[walbourn]

Proposed design of the high-level interface:

class ICompute
{
public:
    virtual ~ICompute() = default;

    virtual void __cdecl Dispatch(
        _In_ ID3D12GraphicsCommandList* commandList,
        uint32_t ThreadGroupCountX,
        uint32_t ThreadGroupCountY,
        uint32_t ThreadGroupCountZ) = 0;
        };
}