Add slang shader for shadow mapping and shadow mapping cascades sample

shaders/slang/shadowmapping/offscreen.slang

@@ -0,0 +1,23 @@
+/* Copyright (c) 2025, Sascha Willems
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ */
+
+struct UBO
+{
+    float4x4 depthMVP;
+};
+ConstantBuffer<UBO> ubo;
+
+[shader("vertex")]
+float4 vertexMain(float3 Pos) : SV_POSITION
+{
+    return mul(ubo.depthMVP, float4(Pos, 1.0));
+}
+
+[shader("fragment")]
+float4 fragmentMain()
+{
+	return float4(1.0, 0.0, 0.0, 1.0);
+}