refactor: externalize builtin forward-lit shader asset

engine/assets/builtin/shaders/forward-lit/forward-lit.frag.glsl

@@ -0,0 +1,36 @@
+// XC_BUILTIN_FORWARD_LIT_OPENGL_PS
+#version 430
+layout(binding = 1) uniform sampler2D uBaseColorTexture;
+
+layout(std140, binding = 1) uniform PerObjectConstants {
+    mat4 gProjectionMatrix;
+    mat4 gViewMatrix;
+    mat4 gModelMatrix;
+    mat4 gNormalMatrix;
+    vec4 gMainLightDirectionAndIntensity;
+    vec4 gMainLightColorAndFlags;
+};
+
+layout(std140, binding = 2) uniform MaterialConstants {
+    vec4 gBaseColorFactor;
+};
+
+in vec3 vNormalWS;
+in vec2 vTexCoord;
+
+layout(location = 0) out vec4 fragColor;
+
+void main() {
+    vec4 baseColor = texture(uBaseColorTexture, vTexCoord) * gBaseColorFactor;
+    if (gMainLightColorAndFlags.w < 0.5) {
+        fragColor = baseColor;
+        return;
+    }
+
+    vec3 normalWS = normalize(vNormalWS);
+    vec3 directionToLightWS = normalize(gMainLightDirectionAndIntensity.xyz);
+    float diffuse = max(dot(normalWS, directionToLightWS), 0.0);
+    vec3 lighting = vec3(0.28) +
+        gMainLightColorAndFlags.rgb * (diffuse * gMainLightDirectionAndIntensity.w);
+    fragColor = vec4(baseColor.rgb * lighting, baseColor.a);
+}