feat(rendering): expose builtin forward scene steps to srp context

managed/XCEngine.ScriptCore/SceneRenderInjectionPoint.cs

@@ -0,0 +1,12 @@
+namespace XCEngine
+{
+    public enum SceneRenderInjectionPoint
+    {
+        BeforeOpaque = 0,
+        AfterOpaque = 1,
+        BeforeSkybox = 2,
+        AfterSkybox = 3,
+        BeforeTransparent = 4,
+        AfterTransparent = 5
+    }
+}