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/terraintessellation/terrain.frag

@@ -0,0 +1,65 @@
+// Copyright 2020 Google LLC
+
+Texture2D textureHeight : register(t1);
+SamplerState samplerHeight : register(s1);
+Texture2DArray textureLayers : register(t2);
+SamplerState samplerLayers : register(s2);
+
+struct DSOutput
+{
+	float4 Pos : SV_POSITION;
+[[vk::location(0)]] float3 Normal : NORMAL0;
+[[vk::location(1)]] float2 UV : TEXCOORD0;
+[[vk::location(2)]] float3 ViewVec : TEXCOORD1;
+[[vk::location(3)]] float3 LightVec : TEXCOORD2;
+[[vk::location(4)]] float3 EyePos : POSITION1;
+[[vk::location(5)]] float3 WorldPos : POSITION0;
+};
+
+float3 sampleTerrainLayer(float2 inUV)
+{
+	// Define some layer ranges for sampling depending on terrain height
+	float2 layers[6];
+	layers[0] = float2(-10.0, 10.0);
+	layers[1] = float2(5.0, 45.0);
+	layers[2] = float2(45.0, 80.0);
+	layers[3] = float2(75.0, 100.0);
+	layers[4] = float2(95.0, 140.0);
+	layers[5] = float2(140.0, 190.0);
+
+	float3 color = float3(0.0, 0.0, 0.0);
+
+	// Get height from displacement map
+	float height = textureHeight.SampleLevel(samplerHeight, inUV, 0.0).r * 255.0;
+
+	for (int i = 0; i < 6; i++)
+	{
+		float range = layers[i].y - layers[i].x;
+		float weight = (range - abs(height - layers[i].y)) / range;
+		weight = max(0.0, weight);
+		color += weight * textureLayers.Sample(samplerLayers, float3(inUV * 16.0, i)).rgb;
+	}
+
+	return color;
+}
+
+float fog(float density, float4 FragCoord)
+{
+	const float LOG2 = -1.442695;
+	float dist = FragCoord.z / FragCoord.w * 0.1;
+	float d = density * dist;
+	return 1.0 - clamp(exp2(d * d * LOG2), 0.0, 1.0);
+}
+
+float4 main(DSOutput input) : SV_TARGET
+{
+	float3 N = normalize(input.Normal);
+	float3 L = normalize(input.LightVec);
+	float3 ambient = float3(0.5, 0.5, 0.5);
+	float3 diffuse = max(dot(N, L), 0.0) * float3(1.0, 1.0, 1.0);
+
+	float4 color = float4((ambient + diffuse) * sampleTerrainLayer(input.UV), 1.0);
+
+	const float4 fogColor = float4(0.47, 0.5, 0.67, 0.0);
+	return lerp(color, fogColor, fog(0.25, input.Pos));
+}