See #10.
Previously, if you called AnchorGlobalAttrs on a ctx with an existing anchor point, it would reset and clear all existing global attrs. This change chooses not to overwrite the existing anchor point, but rather re-use it. I considered creating a new anchor point and copying over fields, but thought the approach of maintaining the previous anchor point would be more common. We can always add a method to reset the anchor point if that is a needed use case.