[D3D12] Output gamma ramp

src/xenia/gpu/d3d12/shaders/stretch.ps.hlsl

@@ -1,5 +1,6 @@
 Texture2D<float4> xe_texture : register(t0);
-SamplerState xe_sampler : register(s0);
+SamplerState xe_sampler_linear_clamp : register(s0);
+
 float4 main(float2 xe_texcoord : TEXCOORD) : SV_Target {
-  return xe_texture.SampleLevel(xe_sampler, xe_texcoord, 0.0f);
+  return xe_texture.SampleLevel(xe_sampler_linear_clamp, xe_texcoord, 0.0f);
 }