Move shaders into glsl and hlsl directories

Move `data/shaders` to `data/shaders/glsl`
Move `data/hlsl` to `data/shaders/hlsl`

Fix up shader paths in the cpp files to point to the new glsl location.

`data/shaders/hlsl/compile.py` still overwrites the glsl .spv files (for
now).

Issue: #723

data/shaders/hlsl/shadowmapomni/offscreen.frag

@@ -0,0 +1,14 @@
+// Copyright 2020 Google LLC
+
+struct VSOutput
+{
+[[vk::location(0)]] float4 Pos : POSITION0;
+[[vk::location(1)]] float3 LightPos : POSITION1;
+};
+
+float main(VSOutput input) : SV_TARGET
+{
+	// Store distance to light as 32 bit float value
+    float3 lightVec = input.Pos.xyz - input.LightPos;
+    return length(lightVec);
+}