Add builtin unlit surface path

engine/assets/builtin/shaders/unlit/unlit.ps.hlsl

@@ -0,0 +1,25 @@
+// XC_BUILTIN_UNLIT_D3D12_PS
+Texture2D gBaseColorTexture : register(t1);
+SamplerState gLinearSampler : register(s1);
+
+cbuffer PerObjectConstants : register(b1) {
+    float4x4 gProjectionMatrix;
+    float4x4 gViewMatrix;
+    float4x4 gModelMatrix;
+    float4x4 gNormalMatrix;
+    float4 gMainLightDirectionAndIntensity;
+    float4 gMainLightColorAndFlags;
+};
+
+cbuffer MaterialConstants : register(b2) {
+    float4 gBaseColorFactor;
+};
+
+struct PSInput {
+    float4 position : SV_POSITION;
+    float2 texcoord : TEXCOORD0;
+};
+
+float4 MainPS(PSInput input) : SV_TARGET {
+    return gBaseColorTexture.Sample(gLinearSampler, input.texcoord) * gBaseColorFactor;
+}