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/skeletalanimation/texture.frag

@@ -0,0 +1,31 @@
+// Copyright 2020 Google LLC
+
+Texture2D textureColorMap : register(t1);
+SamplerState samplerColorMap : register(s1);
+
+struct VSOutput
+{
+[[vk::location(0)]] float2 UV : TEXCOORD0;
+[[vk::location(1)]] float3 Normal : NORMAL0;
+[[vk::location(2)]] float3 ViewVec : TEXCOORD1;
+[[vk::location(3)]] float3 LightVec : TEXCOORD2;
+};
+
+float4 main(VSOutput input) : SV_TARGET
+{
+	float4 color = textureColorMap.Sample(samplerColorMap, input.UV);
+
+	float distSqr = dot(input.LightVec, input.LightVec);
+	float3 lVec = input.LightVec * rsqrt(distSqr);
+
+	const float attInvRadius = 1.0/5000.0;
+	float atten = max(clamp(1.0 - attInvRadius * sqrt(distSqr), 0.0, 1.0), 0.0);
+
+	// Fake drop shadow
+	const float shadowInvRadius = 1.0/2500.0;
+	float dropshadow = max(clamp(1.0 - shadowInvRadius * sqrt(distSqr), 0.0, 1.0), 0.0);
+
+	float4 outFragColor = float4(color.rgba * (1.0 - dropshadow));
+	outFragColor.rgb *= atten;
+	return outFragColor;
+}