[D3D12] Window output

src/xenia/gpu/d3d12/shaders/stretch.ps.hlsl

@@ -0,0 +1,5 @@
+Texture2D<float4> xe_texture : register(t0);
+SamplerState xe_sampler : register(s0);
+float4 main(float2 xe_texcoord : TEXCOORD) : SV_Target {
+  return xe_texture.SampleLevel(xe_sampler, xe_texcoord, 0.0f);
+}