Fix clash when Snow loads in same origin realm and is also called within it (continue #97)

[weizman]

attempt to fix an extension of the problem introduced at https://github.com/LavaMoat/snow/pull/97#issuecomment-1637052914

• define SNOW prop for same window, not top
• make sure that if Snow was already called once on top, it shouldn't reapply all patches, but only call the new cb on top