[Sg] Fix broken Ag rule for FaceVertexCount

See: c149c65

Author: hyazinthh

RELEASE_NOTES.md

@@ -1,4 +1,5 @@
 - Fixed support for 64-bit attributes and uniforms
+- [Sg] Fixed broken Ag rule for `FaceVertexCount`
 
 ### 5.6.4
 - [GL] Dispose MultimediaTimer in LodRenderer to avoid resource exhaustion

src/Aardvark.SceneGraph/Semantics/Attributes.fs

@@ -36,7 +36,9 @@ module AttributeSemantics =
             if scope.VertexIndexBuffer.IsNone then
                 match Map.tryFind DefaultSemantic.Positions app.Values with
                 | Some positions -> app.Child?FaceVertexCount <- BufferView.getCount positions
-                | _ -> ()
+                | _ -> app.Child?FaceVertexCount <- scope.FaceVertexCount
+            else
+                app.Child?FaceVertexCount <- scope.FaceVertexCount
 
         member x.InstanceAttributes(root : Root<ISg>, scope : Ag.Scope) = 
             root.Child?InstanceAttributes <- Map.empty<Symbol, BufferView>