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/shadowmapping/quad.frag

@@ -0,0 +1,18 @@
+// Copyright 2020 Google LLC
+
+Texture2D textureColor : register(t1);
+SamplerState samplerColor : register(s1);
+
+float LinearizeDepth(float depth)
+{
+  float n = 1.0; // camera z near
+  float f = 128.0; // camera z far
+  float z = depth;
+  return (2.0 * n) / (f + n - z * (f - n));
+}
+
+float4 main([[vk::location(0)]] float2 inUV : TEXCOORD0) : SV_TARGET
+{
+	float depth = textureColor.Sample(samplerColor, inUV).r;
+	return float4((1.0-LinearizeDepth(depth)).xxx, 1.0);
+}