Add slang shader for omni-directional shadow mapping sample

shaders/slang/shadowmappingomni/offscreen.slang

@@ -0,0 +1,45 @@
+/* Copyright (c) 2025, Sascha Willems
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ */
+
+struct VSInput
+{
+    float3 Pos;
+};
+
+struct VSOutput
+{
+    float4 Pos : SV_POSITION;
+    float4 WorldPos;
+    float3 LightPos;
+};
+
+struct UBO
+{
+    float4x4 projection;
+    float4x4 view;
+    float4x4 model;
+    float4 lightPos;
+};
+ConstantBuffer<UBO> ubo;
+
+[shader("vertex")]
+VSOutput vertexMain(VSInput input, uniform float4x4 view)
+{
+    VSOutput output;
+    output.Pos = mul(ubo.projection, mul(view, mul(ubo.model, float4(input.Pos, 1.0))));
+
+    output.WorldPos = float4(input.Pos, 1.0);
+    output.LightPos = ubo.lightPos.xyz;
+    return output;
+}
+
+[shader("fragment")]
+float fragmentMain(VSOutput input)
+{
+	// Store distance to light as 32 bit float value
+    float3 lightVec = input.WorldPos.xyz - input.LightPos;
+    return length(lightVec);
+}