Add 3DGS D3D12 composite debug checkpoint

MVS/3DGS-D3D12/shaders/CompositeVS.hlsl

@@ -0,0 +1,12 @@
+struct VertexOutput
+{
+    float4 position : SV_Position;
+};
+
+VertexOutput MainVS(uint vertexId : SV_VertexID)
+{
+    VertexOutput output = (VertexOutput)0;
+    float2 quadPosition = float2(vertexId & 1, (vertexId >> 1) & 1) * 4.0 - 1.0;
+    output.position = float4(quadPosition, 1.0, 1.0);
+    return output;
+}