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/shadowmappingcascade/debugshadowmap.frag

@@ -0,0 +1,16 @@
+// Copyright 2020 Google LLC
+
+Texture2DArray shadowMapTexture : register(t1);
+SamplerState shadowMapSampler : register(s1);
+
+struct VSOutput
+{
+[[vk::location(0)]] float2 UV : TEXCOORD0;
+[[vk::location(1)]] uint CascadeIndex : TEXCOORD1;
+};
+
+float4 main(VSOutput input) : SV_TARGET
+{
+	float depth = shadowMapTexture.Sample(shadowMapSampler, float3(input.UV, float(input.CascadeIndex))).r;
+	return float4(depth.xxx, 1.0);
+}