Stabilize 3DGS D3D12 phase 3 and sort key setup

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

@@ -0,0 +1,43 @@
+#define GROUP_SIZE 64
+
+cbuffer FrameConstants : register(b0)
+{
+    float4x4 gViewProjection;
+    float4x4 gView;
+    float4x4 gProjection;
+    float4 gCameraWorldPos;
+    float4 gScreenParams;
+    float4 gSettings;
+};
+
+ByteAddressBuffer gPositions : register(t0);
+StructuredBuffer<uint> gOrderBuffer : register(t1);
+RWStructuredBuffer<uint> gSortKeys : register(u0);
+
+float3 LoadFloat3(ByteAddressBuffer buffer, uint byteOffset)
+{
+    return asfloat(buffer.Load3(byteOffset));
+}
+
+uint FloatToSortableUint(float value)
+{
+    uint bits = asuint(value);
+    uint mask = (0u - (bits >> 31)) | 0x80000000u;
+    return bits ^ mask;
+}
+
+[numthreads(GROUP_SIZE, 1, 1)]
+void MainCS(uint3 dispatchThreadId : SV_DispatchThreadID)
+{
+    uint index = dispatchThreadId.x;
+    uint splatCount = (uint)gSettings.x;
+    if (index >= splatCount)
+    {
+        return;
+    }
+
+    uint splatIndex = gOrderBuffer[index];
+    float3 position = LoadFloat3(gPositions, splatIndex * 12);
+    float3 viewPosition = mul(float4(position, 1.0), gView).xyz;
+    gSortKeys[index] = FloatToSortableUint(viewPosition.z);
+}