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/imgui/ui.frag

@@ -0,0 +1,15 @@
+// Copyright 2020 Google LLC
+
+Texture2D fontTexture : register(t0);
+SamplerState fontSampler : register(s0);
+
+struct VSOutput
+{
+[[vk::location(0)]] float2 UV : TEXCOORD0;
+[[vk::location(1)]] float4 Color : COLOR0;
+};
+
+float4 main(VSOutput input) : SV_TARGET
+{
+	return input.Color * fontTexture.Sample(fontSampler, input.UV);
+}