Rough impl of IGNORE NULLS for window functions

Hi all,

I wrote an implementation of last_value that ignores null values,
effectively achieving the behavior of last_value(<exp> ignore nulls).
The code is up on BitBucket [1] for the moment.  Thoughts:

* This isn't on the TODO [2].  Is anyone interested in getting this in
the language? I use this feature extensively and need it for an
Oracle port.
* I'd love to get a critique, especially on using WinSetMarkPosition.
* I have not extended the syntax (I simply called the function
last_value_ignore_nulls), and I hope to get some guidance on starting
that.

I'll be starting the other functions shortly.

Cheers,

Joe

[1] https://bitbucket.org/joeb/pg-window-ignore-nulls/src
[2] https://wiki.postgresql.org/wiki/Todo#Window_Functions