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/hlsl/computenbody/particle_integrate.comp

@@ -1,28 +0,0 @@
-// Copyright 2020 Google LLC
-
-struct Particle
-{
-	float4 pos;
-	float4 vel;
-};
-
-// Binding 0 : Position storage buffer
-RWStructuredBuffer<Particle> particles : register(u0);
-
-struct UBO
-{
-	float deltaT;
-	int particleCount;
-};
-
-cbuffer ubo : register(b1) { UBO ubo; }
-
-[numthreads(256, 1, 1)]
-void main(uint3 GlobalInvocationID : SV_DispatchThreadID)
-{
-	int index = int(GlobalInvocationID.x);
-	float4 position = particles[index].pos;
-	float4 velocity = particles[index].vel;
-	position += ubo.deltaT * velocity;
-	particles[index].pos = position;
-}