Request for Patch Feedback: Lag & Lead Window Functions Can Ignore Nulls

> The SQL standard defines a RESPECT NULLS or IGNORE NULLS option for lead,
lag, [...]. This is not implemented in PostgreSQL
(http://www.postgresql.org/docs/devel/static/functions-window.html)
I've had a go at implementing this, and I've attached the resulting patch.
It's not finished yet, but I was hoping to find out if my solution is along
the right lines.

In particular, I'm storing the ignore-nulls flag in the frameOptions of a
window function definition, and am adding a function to the windowapi.h to
get at these options. I'm keeping the last non-null value in
WinGetPartitionLocalMemory (which I hope is the right place), but I'm not
using any of the *GetDatum macros to access it.

An example of my change's behaviour:

nwhite=# select *, lag(num,0) ignore nulls over (order by generate_series)
from
nwhite-# (select generate_series from generate_series(0,10)) s
nwhite-# left outer join
nwhite-# numbers n
nwhite-# on (s.generate_series = n.num);
generate_series | num | lag
-----------------+-----+-----
0 | |
1 | 1 | 1
2 | | 1
3 | | 1
4 | 4 | 4
5 | 5 | 5
6 | | 5
7 | | 5
8 | | 5
9 | 9 | 9
10 | | 9
(11 rows)

I'd find this feature really useful, so I hope you can help me get my patch
to a contributable state.

Thanks -

Nick