[D3D12] Window output

src/xenia/gpu/d3d12/shaders/fullscreen.vs.hlsl

@@ -0,0 +1,7 @@
+// A triangle covering the whole viewport.
+void main(uint xe_vertex_id : SV_VertexID, out float2 xe_texcoord : TEXCOORD,
+          out float4 xe_position : SV_Position) {
+  xe_texcoord = float2(uint2(xe_vertex_id, xe_vertex_id << 1u) & 2u);
+  xe_position =
+      float4(xe_texcoord * float2(2.0, -2.0) + float2(-1.0, 1.0), 0.0, 1.0);
+}