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/shadowmapping/offscreen.vert

@@ -0,0 +1,13 @@
+// Copyright 2020 Google LLC
+
+struct UBO
+{
+	float4x4 depthMVP;
+};
+
+cbuffer ubo : register(b0) { UBO ubo; }
+
+float4 main([[vk::location(0)]] float3 Pos : POSITION0) : SV_POSITION
+{
+	return mul(ubo.depthMVP, float4(Pos, 1.0));
+}