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/mirror.frag

@@ -0,0 +1,42 @@
+// Copyright 2020 Google LLC
+
+Texture2D textureColor : register(t1);
+SamplerState samplerColor : register(s1);
+
+struct VSOutput
+{
+[[vk::location(0)]] float2 UV : TEXCOORD0;
+[[vk::location(1)]] float4 ProjCoord : POSITION0;
+};
+
+float4 main(VSOutput input, bool FrontFacing : SV_IsFrontFace) : SV_TARGET
+{
+	float4 tmp = (1.0 / input.ProjCoord.w).xxxx;
+	float4 projCoord = input.ProjCoord * tmp;
+
+	// Scale and bias
+	projCoord += float4(1.0, 1.0, 1.0, 1.0);
+	projCoord *= float4(0.5, 0.5, 0.5, 0.5);
+
+	// Slow single pass blur
+	// For demonstration purposes only
+	const float blurSize = 1.0 / 512.0;
+
+	float4 color = float4(float3(0.0, 0.0, 0.0), 1.);
+
+	if (FrontFacing)
+	{
+		// Only render mirrored scene on front facing (upper) side of mirror surface
+		float4 reflection = float4(0.0, 0.0, 0.0, 0.0);
+		for (int x = -3; x <= 3; x++)
+		{
+			for (int y = -3; y <= 3; y++)
+			{
+				reflection += textureColor.Sample(samplerColor, float2(projCoord.x + x * blurSize, projCoord.y + y * blurSize)) / 49.0;
+			}
+		}
+		color += reflection;
+	}
+
+	return color;
+}