r112171

[Dmitry Antipov]

To whom it may concern, massive window-related change is commited as r112171.
There was a strong suggestion to enforce easserts in WINDOW_xxx_COMBINATION_P
macros (see 
http://lists.gnu.org/archive/html/emacs-devel/2013-03/msg00696.html),
and I realize all of the reasons around it; on the other side, this requires
even more changes to window management code and a lot more debugging. Given
the fact that this change is quite large already, I believe that an attempt to
fix all glitches and corner cases of window-related code at once may create
very unexpected bugs and more ugly things than this change tries to eliminate.
Therefore now I'm using a "more relaxed" design, assuming that stronger internal
checking may be added step by step and used to find possible subtle bugs.

Dmitry