Add gaussian splat compute shader contracts

engine/assets/builtin/shaders/gaussian-splat-utilities.shader

@@ -0,0 +1,56 @@
+Shader "Builtin Gaussian Splat Utilities"
+{
+    HLSLINCLUDE
+    uint FloatToSortableUint(float value)
+    {
+        const uint rawValue = asuint(value);
+        const uint mask = (rawValue & 0x80000000u) != 0u ? 0xffffffffu : 0x80000000u;
+        return rawValue ^ mask;
+    }
+    ENDHLSL
+
+    SubShader
+    {
+        Pass
+        {
+            Name "GaussianSplatPrepareOrder"
+            HLSLPROGRAM
+            #pragma target 4.5
+            #pragma compute GaussianSplatPrepareOrderCS
+
+            cbuffer PerObjectConstants
+            {
+                float4x4 gProjectionMatrix;
+                float4x4 gViewMatrix;
+                float4x4 gModelMatrix;
+                float4 gCameraRight;
+                float4 gCameraUp;
+            };
+
+            StructuredBuffer<float3> GaussianSplatPositions;
+            RWStructuredBuffer<uint> GaussianSplatSortDistances;
+            RWStructuredBuffer<uint> GaussianSplatOrderBuffer;
+
+            [numthreads(64, 1, 1)]
+            void GaussianSplatPrepareOrderCS(uint3 dispatchThreadId : SV_DispatchThreadID)
+            {
+                uint splatCount = 0u;
+                GaussianSplatOrderBuffer.GetDimensions(splatCount);
+
+                const uint index = dispatchThreadId.x;
+                if (index >= splatCount)
+                {
+                    return;
+                }
+
+                GaussianSplatOrderBuffer[index] = index;
+
+                const float3 localCenter = GaussianSplatPositions[index];
+                const float3 viewCenter =
+                    mul(gViewMatrix, mul(gModelMatrix, float4(localCenter, 1.0))).xyz;
+                GaussianSplatSortDistances[index] = FloatToSortableUint(viewCenter.z);
+            }
+            ENDHLSL
+        }
+    }
+}