dds_get_outer AddRefs the inner surface when it creates the wrapper
surface. The wrapper releases only one reference when it is destroyed,
leaking one of the two. AddRefing in dds_get_outer makes sense because
this is also used by e.g. GetAttachedSurface, where it creates a new
wrapper for surfaces ddrawex didn't know about yet.

Tag1/tag2 are "error tags". You set them with SetTags(), and if a subsequent
drawing operation generates an error, EndDraw() will return the error code and
tags that were set when the error occurred.