this allows callers of Buildings::setSize() to "pre-initialize" the
extents to declare non-rectangular structures. this allows quickfort to
create non-rectangular stockpiles, farm plots, zones, etc. the extents
are still reset as before if the size of the building doesn't match the
caller's expectations.
this commit also fixes a memory leak when setSize() allocates memory for
extents, but the memory is not deallocated if the building is ultimately
invalid for some reason.
3ba984c22c