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/computenbody/particle.vert

@@ -0,0 +1,41 @@
+// Copyright 2020 Google LLC
+
+struct VSInput
+{
+[[vk::location(0)]] float4 Pos : POSITION0;
+[[vk::location(1)]] float4 Vel : TEXCOORD0;
+};
+
+struct VSOutput
+{
+	float4 Pos : SV_POSITION;
+[[vk::location(0)]] float GradientPos : POSITION0;
+[[vk::location(1)]] float2 CenterPos : POSITION1;
+[[vk::builtin("PointSize")]] float PSize : PSIZE;
+[[vk::location(2)]] float PointSize : TEXCOORD0;
+};
+
+struct UBO
+{
+	float4x4 projection;
+	float4x4 modelview;
+	float2 screendim;
+};
+
+cbuffer ubo : register(b2) { UBO ubo; }
+
+VSOutput main (VSInput input)
+{
+	VSOutput output = (VSOutput)0;
+	const float spriteSize = 0.005 * input.Pos.w; // Point size influenced by mass (stored in input.Pos.w);
+
+	float4 eyePos = mul(ubo.modelview, float4(input.Pos.x, input.Pos.y, input.Pos.z, 1.0));
+	float4 projectedCorner = mul(ubo.projection, float4(0.5 * spriteSize, 0.5 * spriteSize, eyePos.z, eyePos.w));
+	output.PSize = output.PointSize = clamp(ubo.screendim.x * projectedCorner.x / projectedCorner.w, 1.0, 128.0);
+
+	output.Pos = mul(ubo.projection, eyePos);
+	output.CenterPos = ((output.Pos.xy / output.Pos.w) + 1.0) * 0.5 * ubo.screendim;
+
+	output.GradientPos = input.Vel.w;
+	return output;
+}