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/shadowmapomni/offscreen.frag

@@ -0,0 +1,14 @@
+// Copyright 2020 Google LLC
+
+struct VSOutput
+{
+[[vk::location(0)]] float4 Pos : POSITION0;
+[[vk::location(1)]] float3 LightPos : POSITION1;
+};
+
+float main(VSOutput input) : SV_TARGET
+{
+	// Store distance to light as 32 bit float value
+    float3 lightVec = input.Pos.xyz - input.LightPos;
+    return length(lightVec);
+}