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.tesc

@@ -0,0 +1,141 @@
+// Copyright 2020 Google LLC
+
+struct UBO
+{
+	float4x4 projection;
+	float4x4 modelview;
+	float4 lightPos;
+	float4 frustumPlanes[6];
+	float displacementFactor;
+	float tessellationFactor;
+	float2 viewportDim;
+	float tessellatedEdgeSize;
+};
+cbuffer ubo : register(b0) { UBO ubo; };
+
+Texture2D textureHeight : register(t1);
+SamplerState samplerHeight : register(s1);
+
+struct VSOutput
+{
+	float4 Pos : SV_POSITION;
+[[vk::location(0)]] float3 Normal : NORMAL0;
+[[vk::location(1)]] float2 UV : TEXCOORD0;
+};
+
+struct HSOutput
+{
+[[vk::location(2)]]	float4 Pos : SV_POSITION;
+[[vk::location(0)]] float3 Normal : NORMAL0;
+[[vk::location(1)]] float2 UV : TEXCOORD0;
+};
+
+struct ConstantsHSOutput
+{
+    float TessLevelOuter[4] : SV_TessFactor;
+    float TessLevelInner[2] : SV_InsideTessFactor;
+};
+
+// Calculate the tessellation factor based on screen space
+// dimensions of the edge
+float screenSpaceTessFactor(float4 p0, float4 p1)
+{
+	// Calculate edge mid point
+	float4 midPoint = 0.5 * (p0 + p1);
+	// Sphere radius as distance between the control points
+	float radius = distance(p0, p1) / 2.0;
+
+	// View space
+	float4 v0 = mul(ubo.modelview, midPoint);
+
+	// Project into clip space
+	float4 clip0 = mul(ubo.projection, (v0 - float4(radius, float3(0.0, 0.0, 0.0))));
+	float4 clip1 = mul(ubo.projection, (v0 + float4(radius, float3(0.0, 0.0, 0.0))));
+
+	// Get normalized device coordinates
+	clip0 /= clip0.w;
+	clip1 /= clip1.w;
+
+	// Convert to viewport coordinates
+	clip0.xy *= ubo.viewportDim;
+	clip1.xy *= ubo.viewportDim;
+
+	// Return the tessellation factor based on the screen size
+	// given by the distance of the two edge control points in screen space
+	// and a reference (min.) tessellation size for the edge set by the application
+	return clamp(distance(clip0, clip1) / ubo.tessellatedEdgeSize * ubo.tessellationFactor, 1.0, 64.0);
+}
+
+// Checks the current's patch visibility against the frustum using a sphere check
+// Sphere radius is given by the patch size
+bool frustumCheck(float4 Pos, float2 inUV)
+{
+	// Fixed radius (increase if patch size is increased in example)
+	const float radius = 8.0f;
+	float4 pos = Pos;
+	pos.y -= textureHeight.SampleLevel(samplerHeight, inUV, 0.0).r * ubo.displacementFactor;
+
+	// Check sphere against frustum planes
+	for (int i = 0; i < 6; i++) {
+		if (dot(pos, ubo.frustumPlanes[i]) + radius < 0.0)
+		{
+			return false;
+		}
+	}
+	return true;
+}
+
+ConstantsHSOutput ConstantsHS(InputPatch<VSOutput, 4> patch)
+{
+    ConstantsHSOutput output = (ConstantsHSOutput)0;
+
+	if (!frustumCheck(patch[0].Pos, patch[0].UV))
+	{
+		output.TessLevelInner[0] = 0.0;
+		output.TessLevelInner[1] = 0.0;
+		output.TessLevelOuter[0] = 0.0;
+		output.TessLevelOuter[1] = 0.0;
+		output.TessLevelOuter[2] = 0.0;
+		output.TessLevelOuter[3] = 0.0;
+	}
+	else
+	{
+		if (ubo.tessellationFactor > 0.0)
+		{
+			output.TessLevelOuter[0] = screenSpaceTessFactor(patch[3].Pos, patch[0].Pos);
+			output.TessLevelOuter[1] = screenSpaceTessFactor(patch[0].Pos, patch[1].Pos);
+			output.TessLevelOuter[2] = screenSpaceTessFactor(patch[1].Pos, patch[2].Pos);
+			output.TessLevelOuter[3] = screenSpaceTessFactor(patch[2].Pos, patch[3].Pos);
+			output.TessLevelInner[0] = lerp(output.TessLevelOuter[0], output.TessLevelOuter[3], 0.5);
+			output.TessLevelInner[1] = lerp(output.TessLevelOuter[2], output.TessLevelOuter[1], 0.5);
+		}
+		else
+		{
+			// Tessellation factor can be set to zero by example
+			// to demonstrate a simple passthrough
+			output.TessLevelInner[0] = 1.0;
+			output.TessLevelInner[1] = 1.0;
+			output.TessLevelOuter[0] = 1.0;
+			output.TessLevelOuter[1] = 1.0;
+			output.TessLevelOuter[2] = 1.0;
+			output.TessLevelOuter[3] = 1.0;
+		}
+	}
+
+    return output;
+}
+
+[domain("quad")]
+[partitioning("integer")]
+[outputtopology("triangle_cw")]
+[outputcontrolpoints(4)]
+[patchconstantfunc("ConstantsHS")]
+[maxtessfactor(20.0f)]
+HSOutput main(InputPatch<VSOutput, 4> patch, uint InvocationID : SV_OutputControlPointID)
+{
+	HSOutput output = (HSOutput)0;
+	output.Pos = patch[InvocationID].Pos;
+	output.Normal = patch[InvocationID].Normal;
+	output.UV = patch[InvocationID].UV;
+	return output;
+}