Add DXGI Adapter Removal Support Test (microsoft#481)

* Add DXGIAdapterRemovalSupportTest which is referenced by the xGPU sample.

* Move Tools dir to the root

Add 'export' keyword for lib_6_3 compatibility (microsoft#471)

In a previous commit, this branch was updated to target lib_6_3 from lib_6_1.
6_3 differs from 6_1 in that it doesn’t export all functions by default. You need to either use ‘export’ (the keyword) or use the -default-linkage option. This change adds the 'export' keyword where necessary.

Merge branch 'develop-dxr'

Merge pull request microsoft#403 from Microsoft/develop-dxr

Adding initial Fallback Layer support for cheap AABB refitting when update build flag is used
Triangle intersection fixes for close geometry
Fallback layer support for geometry flags
Small sample fixes

Updated main readme. (microsoft#368)

Update version in DXR binaries check. (microsoft#361)

Update miniengine and raytracing samples to 17134 (microsoft#360)

Update to a newer d3dx12.h

Adding support for Shader Records with Descriptor Tables and Empty To…

…p Level Acceleration Structures (microsoft#297)

This adds support for local root signatures/shader records with descriptor table handles as the parameter. This removes the need for the app having to do it's own bindless indexing to associate a ClosestHit shader with a texture.

Also adding tests for verifying the use of descriptor tables. These tests have a pass-through raygen trace against a null acceleration structure and invoke the varying miss shaders that test different varieties of descriptor tables pulled from the shader record.

Finally, modifying the MiniEngine sample to use descriptor tables as local root signature arguments, which cleans up the shader code nicely.

Publish DXR Samples and Raytracing Fallback Layer (microsoft#291)

Publishing the DXR Samples and Raytracing Fallback Layer.

There's 3 primary components:
 * The D3D12 Raytracing Fallback Layer is being placed in it's entirety in libraries/D3D12RaytracingFallbac
 * The DXR Samples are being placed in Samples/Desktop/D3D12Raytracing/src
 * A modified version of the MiniEngine's ModelViewer called the D3D12RaytracingMiniEngineSample. It uses the Fallback Layer for a variety of raytracing effects on the Sponza scene.

Users looking to get started can open up the project in Samples\Desktop\D3D12Raytracing\src\D3D12Raytracing.sln

Users will need to also overlay dxcompiler.dll binaries with the samples. These will be provided in the DirectX-Graphics-Samples via the releases tab in regular drops. The drop corresponding to this commit is DirectXRaytracingBinariesV1.0.zip. A script has been integrated into the solution that makes sure users are informed when these are missing and where to get them.