Add shaders re-implemented in HLSL

These were written against the shaders at revision eddd724e7.
There have been changes made since then, which will need to be mirrored.

See `data/hlsl/README.md` for the current status of each sample.

data/hlsl/nv_ray_tracing_reflections/miss.rmiss

@@ -0,0 +1,25 @@
+// Copyright 2020 Google LLC
+
+struct RayPayload {
+	float3 color;
+	float distance;
+	float3 normal;
+	float reflector;
+};
+
+[shader("miss")]
+void main(inout RayPayload rayPayload)
+{
+	float3 worldRayDirection = WorldRayDirection();
+
+	// View-independent background gradient to simulate a basic sky background
+	const float3 gradientStart = float3(0.5, 0.6, 1.0);
+	const float3 gradientEnd = float3(1.0, 1.0, 1.0);
+	float3 unitDir = normalize(worldRayDirection);
+	float t = 0.5 * (unitDir.y + 1.0);
+	rayPayload.color = (1.0-t) * gradientStart + t * gradientEnd;
+
+	rayPayload.distance = -1.0f;
+	rayPayload.normal = float3(0, 0, 0);
+	rayPayload.reflector = 0.0f;
+}