Cursor-extension bug?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

There is a bug in some X servers, in how they handle pixmap cursors
created with a mask of None.

I'd be interested if people could test to see how widespread this is.
I've written a tiny little test program; see
ftp.rodents-montreal.org:/mouse/misc/cursor-bug.c for more (the header
comment elaborates).

I suspect this is present upstream; I've seen it on Linux as well as
NetBSD.  If it's still present and people think it's worth doing, I can
write a PR for it.

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