[Vulkan] Remove old Vulkan code, change shaders directory, create empty Vulkan backend

src/xenia/gpu/shaders/stretch_gamma.ps.hlsl

@@ -0,0 +1,19 @@
+Texture2D<float4> xe_texture : register(t0);
+Texture1D<float3> xe_gamma_ramp : register(t1);
+SamplerState xe_sampler_linear_clamp : register(s0);
+cbuffer XeStretchGammaRootConstants : register(b0) {
+  float xe_gamma_ramp_inv_size;
+};
+
+float4 main(float2 xe_texcoord : TEXCOORD) : SV_Target {
+  float4 color =
+      xe_texture.SampleLevel(xe_sampler_linear_clamp, xe_texcoord, 0.0f);
+  // The center of the first texel of the LUT contains the value for 0, and the
+  // center of the last texel contains the value for 1.
+  color.rgb = color.rgb * (1.0f - xe_gamma_ramp_inv_size) +
+              (0.5 * xe_gamma_ramp_inv_size);
+  color.r = xe_gamma_ramp.SampleLevel(xe_sampler_linear_clamp, color.r, 0.0f).r;
+  color.g = xe_gamma_ramp.SampleLevel(xe_sampler_linear_clamp, color.g, 0.0f).g;
+  color.b = xe_gamma_ramp.SampleLevel(xe_sampler_linear_clamp, color.b, 0.0f).b;
+  return color;
+}