How to create flipped graphics context for an NSBitmapImageRep?

I want to create an NSGraphicsContext for an NSBitmapImageRep
with a flipped coordinate system, but I can't see any way
to do this. NSGraphicsContext has no method for setting the
isFlipped property, and you only get to specify the flipped
state when creating it from a CGContexRef, not from an
NSBitmapImageRep.

Surely there must be some way to do it? Anyone have any ideas?

-- 
Greg

------------------------------------------------------------------------------
Lotusphere 2011
Register now for Lotusphere 2011 and learn how
to connect the dots, take your collaborative environment
to the next level, and enter the era of Social Business.
http://p.sf.net/sfu/lotusphere-d2d