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/indirectdraw/skysphere.frag

@@ -0,0 +1,11 @@
+// Copyright 2020 Google LLC
+
+Texture2D textureColor : register(t2);
+SamplerState samplerColor : register(s2);
+
+float4 main([[vk::location(0)]] float2 inUV : TEXCOORD0) : SV_TARGET
+{
+	const float4 gradientStart = float4(0.93, 0.9, 0.81, 1.0);
+	const float4 gradientEnd = float4(0.35, 0.5, 1.0, 1.0);
+	return lerp(gradientStart, gradientEnd, min(0.5 - inUV.y, 0.5)/0.15 + 0.5);
+}