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/offscreen/phong.vert

@@ -0,0 +1,42 @@
+// Copyright 2020 Google LLC
+
+struct VSInput
+{
+[[vk::location(0)]] float4 Pos : POSITION0;
+[[vk::location(2)]] float3 Color : COLOR0;
+[[vk::location(3)]] float3 Normal : NORMAL0;
+};
+
+struct UBO
+{
+	float4x4 projection;
+	float4x4 model;
+	float4 lightPos;
+};
+
+cbuffer ubo : register(b0) { UBO ubo; }
+
+struct VSOutput
+{
+	float4 Pos : SV_POSITION;
+	float ClipDistance : SV_ClipDistance0;
+[[vk::location(0)]] float3 Normal : NORMAL0;
+[[vk::location(1)]] float3 Color : COLOR0;
+[[vk::location(2)]] float3 EyePos : POSITION0;
+[[vk::location(3)]] float3 LightVec : TEXCOORD2;
+};
+
+VSOutput main(VSInput input)
+{
+	VSOutput output = (VSOutput)0;
+	output.Normal = input.Normal;
+	output.Color = input.Color;
+	output.Pos = mul(ubo.projection, mul(ubo.model, input.Pos));
+	output.EyePos = mul(ubo.model, input.Pos).xyz;
+	output.LightVec = normalize(ubo.lightPos.xyz - output.EyePos);
+
+	// Clip against reflection plane
+	float4 clipPlane = float4(0.0, -1.0, 0.0, 1.5);
+	output.ClipDistance = dot(input.Pos, clipPlane);
+	return output;
+}