Fix 104730: Suppress using anonymous UV layers for rendering.

When an object has no UV layers and an anonymous UV layer is created,
the anonymous layer gets set as the default (render) layer. This is
very confusing because it then uses a hidden anonmous layer for
rendering. This patch suppresses the usage of anonymous layers for
rendering.

@@ -506,6 +508,8 @@ void CustomData_set_layer_stencil_index(struct CustomData *data, int type, int n
 void CustomData_set_layer_flag(struct CustomData *data, int type, int flag);
 void CustomData_clear_layer_flag(struct CustomData *data, int type, int flag);
 
+

@@ -2689,0 +2690,4 @@
+{
+  const int layer_index = data->typemap[type];
+  BLI_assert(customdata_typemap_is_valid(data));
+  BLI_assert(data->layers[layer_index + n].type == type);