Add slang shaders for additional samples

shaders/slang/indirectdraw/skysphere.slang

@@ -0,0 +1,42 @@
+/* Copyright (c) 2025, Sascha Willems
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ */
+
+struct VSInput
+{
+    float4 Pos;
+    float2 UV;
+};
+
+struct VSOutput
+{
+    float4 Pos : SV_POSITION;
+    float2 UV;
+};
+
+struct UBO
+{
+    float4x4 projection;
+    float4x4 modelview;
+};
+ConstantBuffer<UBO> ubo;
+
+[shader("vertex")]
+VSOutput vertexMain(VSInput input)
+{
+    VSOutput output;
+    output.UV = input.UV;
+    // Skysphere always at center, only use rotation part of modelview matrix
+    output.Pos = mul(ubo.projection, float4(mul((float3x3)ubo.modelview, input.Pos.xyz), 1));
+    return output;
+}
+
+[shader("fragment")]
+float4 fragmentMain(VSOutput input)
+{
+	const float4 gradientStart = float4(0.93, 0.9, 0.81, 1.0);
+	const float4 gradientEnd = float4(0.35, 0.5, 1.0, 1.0);
+	return lerp(gradientStart, gradientEnd, min(0.5 - (input.UV.y + 0.05), 0.5)/0.15 - 0.5);
+}