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/shadowmapping/offscreen.frag

@@ -0,0 +1,6 @@
+// Copyright 2020 Google LLC
+
+float4 main() : SV_TARGET
+{
+	return float4(1.0, 0.0, 0.0, 1.0);
+}