[Asset] Curtainless window terrain

Curtainless window terrain asset at KA101's request.

Not implemented in any existing code because:

A) It would leave windows unable to be barricaded/taped/whatever after tearing down windows
B) Adding another construction option for 'barricade/tape/whatever curtainless window' would be confusing since it's called 'closed window' and 'open window'
C) I didn't want to dive into the code to allow multiple pre-terrain types for one construction because I don't know how (in order to fix B and then indirectly fix A)

I leave this in the hands of more skilled coders...

(#7741)