refactor: externalize builtin forward-lit shader asset

engine/assets/builtin/shaders/forward-lit/forward-lit.ps.hlsl

@@ -0,0 +1,36 @@
+// XC_BUILTIN_FORWARD_LIT_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;
+    float3 normalWS : TEXCOORD0;
+    float2 texcoord : TEXCOORD1;
+};
+
+float4 MainPS(PSInput input) : SV_TARGET {
+    float4 baseColor = gBaseColorTexture.Sample(gLinearSampler, input.texcoord) * gBaseColorFactor;
+    if (gMainLightColorAndFlags.a < 0.5f) {
+        return baseColor;
+    }
+
+    float3 normalWS = normalize(input.normalWS);
+    float3 directionToLightWS = normalize(gMainLightDirectionAndIntensity.xyz);
+    float diffuse = saturate(dot(normalWS, directionToLightWS));
+    float3 lighting = float3(0.28f, 0.28f, 0.28f) +
+        gMainLightColorAndFlags.rgb * (diffuse * gMainLightDirectionAndIntensity.w);
+    return float4(baseColor.rgb * lighting, baseColor.a);
+}