refactor: externalize builtin object id and grid shader assets

engine/assets/builtin/shaders/object-id/object-id.frag.glsl

@@ -0,0 +1,14 @@
+// XC_BUILTIN_OBJECT_ID_OPENGL_PS
+#version 430
+layout(std140, binding = 0) uniform PerObjectConstants {
+    mat4 gProjectionMatrix;
+    mat4 gViewMatrix;
+    mat4 gModelMatrix;
+    vec4 gObjectIdColor;
+};
+
+layout(location = 0) out vec4 fragColor;
+
+void main() {
+    fragColor = gObjectIdColor;
+}